I know you are suppose to use the present verb tense after DID, “ I did do my homework” “ I didn’t go to the park”. But what about: “I DID what you TOLD me?”, I know this is correct, but why? Isn’t it suppose to be: “ I did what you tell me”
The answer depends on how you use the verb did.
If you use it as an auxiliary verb (**did say, did do, did think) the main verb has to be in the base form.
However this is not the case when you use did as the main verb.
In your sentence, did is not an auxiliary verb.
What you are saying really is I did it. But instead of it, the object of did is the noun clause what you told me.
If, for example, you wanted to emphasise that you did it, you would write:
I did do/say what you told me.
Here did acts as the auxiliary verb and do or say is in the base form.
I did do my homework ---> "did (an auxiliary verb)" and "do (the main verb)"
I DID what you TOLD me ---> "DID (the main verb of the main clause, i.e. I DID)", "what you TOLD me (a noun clause)" and "TOLD (the main verb of the noun clause)"
there should be a proportion between the tense of the main clause and the noun clause.
I went before you came.