$1.$ $. . .$

$(-)$ I didn't see him a week ago.

- S + didn't + V (Bare).

$(?)$ When did you see him.

- Did + S + V (Bare)?

- "a week ago": nói về thời gian.

$→$ Dùng "When" để hỏi sau "Did".

$2.$ $. . .$

$(-)$ They don't visit their old friends once a year.

- S + don't/ doesn't + V (Bare).

$(?)$ How often do they visit their old friends?

- Do/ Does + S + V (Bare)?

- "once a year": nói về tần suất.

$→$ Dùng "How often" để hỏi sau "Do".

$3.$ $. . .$

$(-)$ Her children aren't sitting next to theirs.

- S + to be + (not) + V-ing.

$(?)$ Where are her children sitting?

- "next to" chỉ nơi chốn.

$→$ Dùng "Where" để hỏi.

$4.$ $. . .$

$(-)$ Those expensive glasses aren't Paul's.

- S + to be + (not)  + O.

$(?)$ Whose are those expensive glasses?

$5.$ $. . .$

$(-)$ Anne didn't invite Tom to her birthday party.

- S + didn't + V (Bare).

$(?)$ Who did Anna invite to her birthday party?

- Did + S + V (Bare)?

- "Tom": nói về người

$→$ Dùng "Who" để hỏi sau "Did".

$6.$ $. . .$

$(-)$ They didn't buy any Chinese toys.

- S + didn't + V (Bare).

$(?)$ How many Chinese toys did they buy?

- "some": nói về số lượng

$→$ Dùng "How many" để hỏi sau "Did".

$7.$ $. . .$ 

$(-)$ He didn't drink a lot at the party.

- S + didn't + V (Bare).

$(?)$ How much did he drink at the party?

- "a lot" nói về số lượng

$→$ Dùng "How much" để hỏi sau "Did".

$8.$ $. . .$

$(-)$ She didn't cry yesterday because she was unhappy.

- S + didn't + V (Bare).

$(?)$ Why did she cry yesterday?

- "Because": giải thích nguyên nhân.

$→$ Dùng "Why" hỏi lí do.