Viết dạng đúng của động từ trong ngoặc để hoàn chỉnh các câu sau:
1.The post office isn't far from here . I (walk)...............there many time .
2.His father would buy that house if it (not be)................expensive.
3.We (study)...................when our teacher came in the classroom.
4. I used to (take)....................to the zoo by my father when he had free time