Английский язык, 5 предложений, помогите!

1) Tell me at once ____ Margaret arrives.
a) if
b) when
c)in case

2) It is time you___ home, but i'd rather you ___ here.
a)go, stay
b) went, stayed
c) go, stayed
d) went, stay

3) He asked me, if ___ my friends the day before.
a) had visited
b) have visited
c) visited

4) They knew their friends ___ to Kiev then.
a) are going
b) will go

c) were going