1. When I was looking out of the window, I saw John.
2. Have you ever played the piano since you left school?
3. I haven't seen him for two days.
4. When he was trying to open the door, he dropped his key.
5. They have been living in this house since 1987.
6. My father was washing his car while my mother and I were preparing dinner.
7. Did you meet her yesterday?
8. Mary has been in London for three days.
9. The headmaster entered the classroom when they were writing their exams.
10. He bought a new car last week.
11. I think that they will arrive tomorrow morning.
12. Last Wednesday they played chess after they had done their homework.
THE is the definite article. when you have ONE of something that is definite, you use THE. there is only one mississippi river, so you must use 'the'.
'an' is an indefinite article, not appropriate. if you have one of something thats not specific such as 'an apple'.
He will be twenty in a month.
'in a month' says this is in the future, so you need a future tense verb
'am' goes with 'I' always (I am ...)
'is' is present tense, but you could say 'he is going to be twenty in a month'
We might go to Peru next year. We're not sure.
'not sure' means it may not happen. 'might' is also uncertain
so 'should' means we need to do something
that's the woman whose son is an actor.
i am not sure why, just that this one is correct
This afternoon, we are going sandboarding
a) and b) are the same answer
'this afternoon' means future tense
'going to' states a plan or intention, where 'will' is used in a different sense, as in a promise or request for help
i'm bored of camping trips. let's go on a safari
'for' is definitely wrong
i have heard 'from' used, but i suspect it is not proper grammar
извините за все английский, мой руский не достаточно хорошо.