Answer:
1 I talked to him on Monday. --> CAN'T CHANGE
2 I hated school when I was a teenager. --> I used to hate school when I was a teenager.
3 My parents didn't eat out very often. --> My parents didn't use to eat out very often.
4 We visited our grandmother every weekend. --> We used to visit our grandmother every weekend.
5 I went to the cinema with Mark yesterday. --> CAN'T CHANGE
6 Did you watch a lot of TV when you were a child? --> Did you use to watch a lot of TV when you were a child?
Explanation:
First, let's understand why we can't change numbers 1 and 5. "Used to" can be employed to refer to habitual actions in the past, that is, actions that happened frequently. In numbers 1 and 5, we are talking about a single action that happened at a specific time in the past, not about actions that happened often. Therefore, "used to" is not applicable to them.
All the other sentences are speaking of actions that happened often during a period of time in the past. If the sentence is affirmative, all we need to do is add "used to" before the main verb in its base form. If the sentence is negative, we use "didn't use to" before the main verb. Interrogative sentences place the auxiliary "did" before the subject, followed by "use to" and the main verb. Note that, in the negative and interrogative forms, it's "use", not "used".
