viết lại câu sao cho nghĩa ko đổi

1.dogs are difficult to train

- it's

2.they think that learning english is not easy

-they think that it

3.the children couldn't go swimming because the sea was rough

- the sea was so

4.remember to write a letter to me

- don't

5.i often walk from my house to the zoo in fifteen minutes

-it often