:root {
    --primary: #4F46E5;
    --primary-light: #818CF8;
    --secondary: #10B981;
    --dark: #1F2937;
    --darker: #111827;
    --light: #F3F4F6;
    --white: #FFFFFF;
    --gray: #6B7280;
    --border: #E5E7EB;
    --shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
    --shadow-hover: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05);
    --transition: all 0.3s ease;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Poppins', sans-serif;
}

body {
    background-color: var(--light);
    color: var(--dark);
    line-height: 1.6;
    display: flex;
    min-height: 100vh;
}

/* Layout */
.sidebar {
    width: 260px;
    background-color: var(--white);
    height: 100vh;
    position: fixed;
    left: 0;
    top: 0;
    box-shadow: 2px 0 10px rgba(0,0,0,0.05);
    z-index: 100;
    display: flex;
    flex-direction: column;
    transition: var(--transition);
}

.sidebar-header {
    padding: 25px 20px;
    border-bottom: 1px solid var(--border);
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.sidebar-header h2 {
    font-size: 1.3rem;
    color: var(--primary);
    font-weight: 700;
}

.mobile-close {
    display: none;
    background: none;
    border: none;
    font-size: 1.2rem;
    color: var(--gray);
    cursor: pointer;
}

.nav-links {
    list-style: none;
    padding: 20px 0;
}

.nav-links li a {
    display: flex;
    align-items: center;
    padding: 15px 25px;
    color: var(--gray);
    text-decoration: none;
    font-weight: 500;
    transition: var(--transition);
    border-left: 4px solid transparent;
}

.nav-links li a i {
    width: 25px;
    margin-right: 10px;
    font-size: 1.1rem;
}

.nav-links li a:hover {
    background-color: #f8fafc;
    color: var(--primary);
}

.nav-links li a.active {
    background-color: #e0e7ff;
    color: var(--primary);
    border-left-color: var(--primary);
}

.main-content {
    flex: 1;
    margin-left: 260px;
    min-height: 100vh;
    transition: var(--transition);
}

.mobile-header {
    display: none;
    background-color: var(--white);
    padding: 15px 20px;
    box-shadow: var(--shadow);
    align-items: center;
    position: sticky;
    top: 0;
    z-index: 90;
}

.mobile-header button {
    background: none;
    border: none;
    font-size: 1.5rem;
    color: var(--dark);
    margin-right: 15px;
    cursor: pointer;
}

.content-container {
    padding: 40px;
    max-width: 1200px;
    margin: 0 auto;
}

/* Page Sections */
.page-section {
    display: none;
    animation: fadeIn 0.4s ease;
}

.page-section.active {
    display: block;
}

@keyframes fadeIn {
    from { opacity: 0; transform: translateY(10px); }
    to { opacity: 1; transform: translateY(0); }
}

.section-header {
    margin-bottom: 30px;
}

.section-header h1 {
    font-size: 2.5rem;
    color: var(--darker);
    margin-bottom: 10px;
}

.section-header p {
    color: var(--gray);
    font-size: 1.1rem;
}

/* Home Section */
.hero {
    background: linear-gradient(135deg, var(--primary), var(--primary-light));
    border-radius: 20px;
    padding: 60px 40px;
    color: var(--white);
    text-align: center;
    margin-bottom: 40px;
    box-shadow: var(--shadow);
}

.hero h1 {
    font-size: 2.8rem;
    margin-bottom: 15px;
    font-weight: 700;
}

.hero p {
    font-size: 1.1rem;
    max-width: 700px;
    margin: 0 auto 30px auto;
    opacity: 0.9;
}

.btn {
    padding: 12px 25px;
    border-radius: 30px;
    font-weight: 600;
    cursor: pointer;
    border: none;
    transition: var(--transition);
    font-size: 1rem;
}

.btn-primary {
    background-color: var(--white);
    color: var(--primary);
}

.btn-primary:hover {
    background-color: #f8fafc;
    transform: translateY(-2px);
    box-shadow: var(--shadow);
}

.btn-primary:disabled {
    opacity: 0.6;
    cursor: not-allowed;
    transform: none;
}

.btn-secondary {
    background-color: transparent;
    color: var(--white);
    border: 2px solid var(--white);
    margin-left: 15px;
}

.btn-secondary:hover {
    background-color: rgba(255,255,255,0.1);
}

.features {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 20px;
}

.feature-card {
    background-color: var(--white);
    padding: 30px 20px;
    border-radius: 15px;
    text-align: center;
    box-shadow: var(--shadow);
    transition: var(--transition);
}

.feature-card:hover {
    transform: translateY(-5px);
    box-shadow: var(--shadow-hover);
}

.feature-card i {
    font-size: 2.5rem;
    color: var(--primary);
    margin-bottom: 15px;
}

.feature-card h3 {
    margin-bottom: 10px;
}

.feature-card p {
    color: var(--gray);
    font-size: 0.95rem;
}

/* Material Content */
.card-box {
    background-color: var(--white);
    border-radius: 15px;
    padding: 30px;
    margin-bottom: 30px;
    box-shadow: var(--shadow);
}

.card-box h2 {
    color: var(--primary);
    margin-bottom: 15px;
    display: flex;
    align-items: center;
    gap: 10px;
}

.example-box {
    background-color: var(--light);
    border-left: 4px solid var(--primary);
    padding: 20px;
    border-radius: 8px;
    margin-top: 20px;
}

.example-box hr {
    border: none;
    border-top: 1px solid var(--border);
    margin: 15px 0;
}

.example-box ul {
    list-style-position: inside;
    margin-top: 10px;
}

.example-box li {
    margin-bottom: 5px;
}

.grid-2 {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: 20px;
    margin-top: 20px;
}

.info-card {
    background-color: var(--white);
    padding: 20px;
    border-radius: 10px;
    border: 1px solid var(--border);
}

.formula-box {
    background-color: var(--darker);
    color: #a7f3d0;
    padding: 20px;
    border-radius: 10px;
    font-family: monospace;
}

.formula-box ul {
    list-style: none;
}

.formula-box li {
    margin-bottom: 10px;
}

.formula-box strong {
    color: var(--white);
}

.mt-20 { margin-top: 20px; }

/* Tenses Grid (Adapted from original) */
.filters {
    display: flex;
    flex-wrap: wrap;
    gap: 10px;
    margin-bottom: 30px;
}

.filter-btn {
    padding: 8px 16px;
    border: none;
    background-color: var(--white);
    color: var(--gray);
    border-radius: 20px;
    font-weight: 500;
    cursor: pointer;
    box-shadow: var(--shadow);
    transition: var(--transition);
}

.filter-btn:hover, .filter-btn.active {
    background-color: var(--primary);
    color: var(--white);
}

.tenses-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
    gap: 25px;
}

.tense-card {
    background-color: var(--white);
    border-radius: 15px;
    padding: 25px;
    box-shadow: var(--shadow);
    cursor: pointer;
    border-top: 5px solid var(--primary);
    transition: var(--transition);
}

.tense-card:hover {
    transform: translateY(-5px);
    box-shadow: var(--shadow-hover);
}

.card-category { font-size: 0.8rem; text-transform: uppercase; color: var(--primary); font-weight: 600; margin-bottom: 5px; }
.card-title { font-size: 1.3rem; font-weight: 700; margin-bottom: 15px; }
.card-preview { font-size: 0.9rem; color: var(--gray); margin-bottom: 20px; display: -webkit-box; -webkit-line-clamp: 2; -webkit-box-orient: vertical; overflow: hidden; }
.read-more { color: var(--primary); font-weight: 600; font-size: 0.9rem; }

/* Modal */
.modal-overlay {
    position: fixed; top: 0; left: 0; width: 100%; height: 100%;
    background-color: rgba(0,0,0,0.5); z-index: 1000;
    display: flex; justify-content: center; align-items: center;
    opacity: 0; visibility: hidden; transition: var(--transition);
    backdrop-filter: blur(5px);
}

.modal-overlay.active { opacity: 1; visibility: visible; }
.modal-content {
    background-color: var(--white); width: 90%; max-width: 600px;
    border-radius: 20px; padding: 30px; position: relative;
    max-height: 90vh; overflow-y: auto;
}
.close-btn { position: absolute; top: 20px; right: 20px; background: none; border: none; font-size: 1.5rem; cursor: pointer; color: var(--gray); }

/* Quiz */
.quiz-controls {
    display: flex;
    gap: 15px;
    margin-bottom: 30px;
    align-items: center;
}

.quiz-select {
    padding: 10px 15px;
    border-radius: 8px;
    border: 1px solid var(--border);
    font-size: 1rem;
    outline: none;
}

.quiz-select:focus {
    border-color: var(--primary);
}

.quiz-controls .btn-primary {
    background-color: var(--primary);
    color: var(--white);
}

.question-card {
    background-color: var(--white);
    padding: 30px;
    border-radius: 15px;
    box-shadow: var(--shadow);
    margin-bottom: 20px;
}

.question-card h3 {
    font-size: 1.3rem;
    margin-bottom: 20px;
}

.option-btn {
    display: block;
    width: 100%;
    text-align: left;
    padding: 15px 20px;
    margin-bottom: 10px;
    background-color: var(--light);
    border: 2px solid transparent;
    border-radius: 10px;
    cursor: pointer;
    font-size: 1rem;
    transition: var(--transition);
}

.option-btn:hover {
    background-color: #e0e7ff;
}

.option-btn.selected {
    background-color: #e0e7ff;
    border-color: var(--primary);
}

.option-btn.correct {
    background-color: #d1fae5;
    border-color: var(--secondary);
    color: #065f46;
}

.option-btn.wrong {
    background-color: #fee2e2;
    border-color: #ef4444;
    color: #991b1b;
}

.quiz-actions {
    display: flex;
    justify-content: flex-end;
}

.quiz-actions .btn-primary {
    background-color: var(--primary);
    color: var(--white);
}

.quiz-actions .btn-secondary {
    background-color: var(--gray);
    color: var(--white);
    border: none;
}

.quiz-result {
    text-align: center;
    background-color: var(--white);
    padding: 40px;
    border-radius: 15px;
    box-shadow: var(--shadow);
}

.score-circle {
    width: 120px;
    height: 120px;
    border-radius: 50%;
    background: linear-gradient(135deg, var(--primary), var(--primary-light));
    color: var(--white);
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 2rem;
    font-weight: 700;
    margin: 20px auto;
}

.hidden {
    display: none !important;
}

/* Responsive */
@media (max-width: 768px) {
    .sidebar { transform: translateX(-100%); }
    .sidebar.open { transform: translateX(0); }
    .mobile-header { display: flex; }
    .main-content { margin-left: 0; }
    .content-container { padding: 20px; }
    .mobile-close { display: block; }
    .hero { padding: 40px 20px; }
    .hero h1 { font-size: 2rem; }
    .quiz-controls { flex-direction: column; align-items: stretch; }
}

/* Additional Quiz Styles */
.quiz-input {
    width: 100%;
    padding: 15px;
    border: 2px solid var(--border);
    border-radius: 10px;
    font-size: 1rem;
    margin-bottom: 20px;
    outline: none;
    transition: var(--transition);
    font-family: inherit;
}

.quiz-input:focus {
    border-color: var(--primary);
}

.error-word {
    display: inline-block;
    padding: 5px 10px;
    margin: 5px;
    background-color: var(--light);
    border: 2px solid transparent;
    border-radius: 5px;
    cursor: pointer;
    transition: var(--transition);
    font-weight: 500;
}

.error-word:hover {
    background-color: #e0e7ff;
}

.error-word.selected {
    background-color: #e0e7ff;
    border-color: var(--primary);
}

.error-word.correct {
    background-color: #d1fae5;
    border-color: var(--secondary);
    color: #065f46;
}

.error-word.wrong {
    background-color: #fee2e2;
    border-color: #ef4444;
    color: #991b1b;
}

.ref-answer-box {
    background-color: #e0e7ff;
    padding: 20px;
    border-radius: 10px;
    border-left: 4px solid var(--primary);
    margin-top: 20px;
}

.ref-answer-box h4 {
    color: var(--primary);
    margin-bottom: 10px;
}