The ставится в случае:
1- Конретный предмет (I have bought the black camera.)
2- степень превосходства(I have bought the best camera.)
A/an ставится когда внимание обращается не на существительное а на сам факт. (I have bought a camera.(важен факт того,что ты купил камеру.))
Вроде как то так.
1 Where do you live?
2 To whom did you give the book?
3 Who did you lend his car?
4 How long ago did you get married?
5 What game does he play best?
6 Which pullover would you like?
7 What sort of pullovers do you like?
8 Who merdered the president?
9 Whom did a madman murder?
10 What did madman do with the president?
11 How often do you go to the cinema
12 How much does the journey take from here?
13 How do I get to your house?
14 In which box does he keep the key
15 How much was milk here in the morning?
1) happend
2) were sitting
3) caught
4) was getting
5) was having
6) broke
7) hit
8) turned off