I have one question about English grammar

I am not sure why every interaction with you comes down to you bragging about your credentials. I don't mean to be rude, but nearly completing a sociology degree at the University of Derby says nothing about your linguistic skills. Stop making things personal.

Those sample sentences are never spoken by competent speakers of English; they are inherently non-colloquial.

do you even know what colloquial means?

they are spoken by non native speakers of english, thus my point remains. know he much, this boy does? (yoda speak, by defacto the greatest form of english ever invented). but i have already said you are right, so just leave it thhanks.

You asked me a question about my credentials. You imply I don't understand what colloquial means. Then tell me to "just leave it". Make your mind up.

i'm slightly drunk, and very tired with arguing about people who think that somehow their 'English' is better than mine? just leave it

Am I to understand that your English is therefore better than mine?

je ne comprenez tout biens cettes gendre, oui merci. c'est toutes biens!

You clearly a) don't speak French and b) don't understand what I'm saying

You don't mean "true", you mean "correct".

"Do" is used to formulate a question so you can't use it more than once to ask one question. So you can't say: "Do you think does your brother like you?". You'd use one or the other version of "do", so either "Do you think (that).." or "Does your brother.."

Also inversions can be used to formulate a question, but you can't use an inversion as well as "do", you either say "Do you think (that) I am stupid" or "Am I stupid?"

Well put. Also remember this is the same for all auxiliary verbs in all tenses. Not just 'do'. In all indirect questions we use the affirmative for the second part of the question. This is because we only use the question form at the start of a sentence.
I teach English as a foreign language and my students always find it difficult.

For example a more complicated one:
"When did the train leave?" or
"Would you mind me telling me when the train left?"
In the first sentence we use "when did..." as it is the start of the sentence.
While in the second it is nearly like 2 questions in one. "Would you.." is the start of the sentence so we use the question form/inversion. After that we just continue using the affirmative; "when the train left" because it is not the first words of the sentence.

A different example not using an indirect question to prove the rule words in all situations:
"Why isn't he going to the party" and
"He has already told me why he isn't going to the party".
In the first sentence "Why isn't he" is at the start of the sentence so it is in the question form/inversion.
In the second the question is said in the middle of the sentence not at the start of the sentence so it is not in the affirmative/negative form; "why he isn't..."

I hope this makes sense and help understand why and when to use it.

If you have any grammatical questions please I teach all levels from Beginner (A2) to Proficiency (C2) so I am sure it is a question I have been asked before and will be happy to answer.

what is your langue maternelle

I am guessing you mean my 'mother tongue'/'native language' based on that it looks like the Portuguese.
My native language is English. I am purely British. English actually the only language I speak fluently. I speak a little Portuguese because I live in Lisbon but that is all.
Why do you ask?

Because you said you taught, so I thought maybe you taught English as a foreign language

Yeah I do. I teach English as a foreign language to Portuguese people in Lisbon, Portugal. However I am a native teacher. There is a big want for native teachers around the world.

But if you don't really falar português, is that not hard?

I only need a little for the very low levels. I speak enough for that. Many don't even know that. When I arrived I couldn't speak a word.
Even if I could I am not to speak to them in Portuguese. All classes are only done in English. Just the odd translation if they don't understand. Students too aren't allowed to speak Portuguese in class either.
It is apparently the best way to learn.