The formation of regular verbs in Simple Past is very simple. We just add an ED suffix to the end of the verb like these examples:
want – wanted
call – called
like – liked
decide – decided
If the verb is irregular, we use column 2 from the table of irregular verbs. This column contains all the past forms of irregular verbs.
I / you / he, she, it / we / you / they didn’t read
Did I / you / he, she, it / we / you/ they read?
Third conditionals are not tenses; however, they contain tenses, of course. We use this construction to express past conditions which cannot happen anymore. Basically, we can call this phenomenon unreal past because the event or action cannot happen.
The formation is the following: if + Past Perfect (had + verb 3) + would have + verb 3
If my alarm clock had gone off, I would have woken up in time
Since the alarm clock did not go off, we can only talk about this event as a hypothetical event. In the second clause, we can see what could have happened if the condition realises.
But we can also use could or might instead of would:
If you had told me your problems, I could have helped you
If you had invited her to the party, she might have come
COULD expresses the ability in the past which could have happened but it did not. MIGHT represents the chance that something might have happened but it did not.