` 1`. I don't know how to do this exercise.

` -` how to do sth : cách làm gì

` 2`. I have no idea about where to go tonight.

` -` where to do sth : địa điểm, nơi làm gì

` 3`. She doesn't know what to say.

` -` what to do sth : nên làm gì

` 4`. I am thinking about who to make friends.

` -` who to do sth : ai làm gì

` 5`. My father didn't know when to go to the airport.

` -` when to do sth : thời gian, lúc để làm gì