May I know what is the rule on using two past tenses in a sentence? Why are some allowed and others not?
Examples:
Some people told me that double past tense is not allowed, but I keep seeing them. What are the rules?
There is no such thing as "double past tense" the grammar in the two questions is completly different.
In the first there is one clause and one verb phrase "did not tell" This is how English forms the negation of a verb
I tell → I do tell → I do not tell.
He tells → He does tell → He does not tell
I told → I did tell → I did not tell
In all cases the infinitive form of the verb "tell" is used with "do". The word "tell" has no tense when used with do support.
In the second example there are two clauses with two finite verbs.
[I knew] (that) [he was ...]
The tense in the second verb can be the same, or it can be different to the tense in the first. There is no rule in English (or any other language of which I am aware that has verb tense) that the tense in every clause must be the same.
I know (now) that he was dating her (but they broke up last week).
I knew (yesterday) that he will go to...(tomorrow)
There is double past tense and it's always wrong, but then it would have to be like this: I did not told him. For 'tell' is present.
That's why your examples are not double past tense.
did not tell is the simple past tense in the negative. I told him yesterday. I didn't or did not tell him yesterday.
I knew = past tense of know.
I know you are coming to the party. [present tense for both verbs. The present continuous is often used with a future idea.] There are two verbs in the sentence, both in the present tense.
I knew you were coming to the party. [past tense of know and past continuous tense of come] There are two verbs in the sentence both in past tenses.
