Решение:
- My dog is bigger than your dog. (Избегаем двойной степени сравнения).
- This is the easiest task. (Суперлятив 'easiest' не требует 'most').
- She is taller than her sister. (Используем 'than' для сравнения).
- A plane is faster than a train. (Правильная сравнительная степень от 'fast').
Ответ: My dog is bigger than your dog; This is the easiest task; She is taller than her sister; A plane is faster than a train.