`1)` It has been said by the tourists that the best time to visit Son Doong Cave was in March.

`2)` It has been discovered by scientists that there is water on Mars.

`3)` It is known that English is an international language.

`->` It + be + V + (+by O) + (that) + ...

`->` be ở đây là is/has/had

`-`

`1)` They can't decide what to do in this situation.

`2)` I don't know where to go on the weekend.

`3)` She was not sure where to put her mobile phone.

`->` can(not) + V-inf: có/không thể làm gì

`->` do/does (not) + V-inf: (không) làm gì

`->` wh- + to + V-inf

`-`

@magnetic