`1` 20 years had passed since I last saw such a disastrous storm.

`-` QKHT (+): S + had Ved/Vpp

`-` QKĐ (+): S + Ved/V2

`2` No sooner had they entered the hall than it started to rain cats and dogs.

`-` No sooner + had + S + Ved/Vpp + than + S + V​ed/V2

`->` Ngay sau khi...thì...

`-` raining cats and dogs (idiom): mưa rất to

`3` Mai reminded Hung to bring his passports with him when he went abroad.

`-` remind sb to V: nhắc nhở ai đó làm gì

`4` The sooner he stops smoking, the better he'll feel.

`-` So sánh đồng tiến:

`->` The + so sánh hơn + S + V, the + so sánh hơn + S + V