On the first set of questions, question 9 is: You......better take the book back to the library today to avoid paying a fine. The correct answer is had. Also question 20: Its time we........ The correct answer is left. Both answers are past tense and I do not understand why. Could you explain please.
'You had better' is similar to 'you should'.
It's time we + past tense- the past tense is used because they haven't left yet, so it is an imaginary action at the time of speaking, like 'If I were you', and to show that something is an imaginary present, we use the past tense/subjunctive.