"I cannot tell you what we did yesterday."
Translation:Je ne peux pas te raconter ce que nous avons fait hier.
Here, "ce" means "that".
It is followed by a second part of the sentence which is introduced by a relative pronoun "que" (= "which"). FYI: this second part is called "relative clause".
Literally, you could translate the French sentence into:
- "I cannot tell you that which we did yesterday."
But it is more natural to translate it to:
- "I cannot tell you what we did yesterday."
Usually, the past imperfect is used to describe something that used to happen for a long time (ex: "Nous faisions du sport à l'école" for "We practiced sports at school).
In this sentence, you can use either the past imperfect or the compound past. The past imperfect tense implies that you are talking about what people did for a long period during this day (not a precise event, in which case you would use the compound past).
It is paired with "que".
In this sentence, "ce" is the antecedent of the relative pronoun "que".