Lets strat it from the beginning

Nowadays I have been thinkg about the common mistake that I make while posting my questions on this forum. Then I realized that my problem is that I am learning advancfed level english without mastering the basic levels. For example I always say things like;
Does he has?
Does he means?
etc etc

So I thnik that I should strt a thread that will cover such basic rules so that it be helpful for everyone.

Lets start with the problem that I have described above. Although I don't have enough knowledge about grammar, I hope the experts on this forum will help all of us.

Verb To Do
The verb do is the basic form of this verb; did is used to express the past and does to express that the subject is a thrid person singular. With does we cannot use s with the verb. for example
Does he say? (not does he says)
Does he walk? (not does he walks)

The same stands true for negative sentences;
Doesn't he sing (not doesn't he sings)
Does she understand (not doesn't she understands)

Now thats what I knoe wbout does. But I think that the same rule also stands true for did. For example;
Did you get it? (not did you got it?)
Did you say that? (not did you said that?) hmm... Well I am not pretty sure about that.

Please don't put any credence in the did section. Lets wait if any expert can help us.

GB