3. — They _______ (come) back yet? — No, they _______. They _______ (walk) now. — How long _______ they _______ (walk)? — They _______ (walk) since 2 pm.

Решение:

1. — They have come (come) back yet?
2. — No, they haven't come. They are walking (walk) now.
3. — How long have they been walking (walk)?
4. — They have been walking (walk) since 2 pm.

Ответ: have come; haven't come; are walking; have been walking; have been walking.