Question tags are very common in the English language. They are not difficult, but you must follow some simple rules and not get confused.

- You're French, aren't you?

The question tag is the last bit of the sentence: "aren't you?"
We use question tags to keep the conversation going. If I say "you're French", that may be the end of the conversation. You don't have to say anything. If I say "you're French, aren't you?", I'm asking a question and now you should answer, so the conversation keeps going:
- You're French, aren't you?
- Yes, I am

We use a question tag when we are not sure of something, so we ask a question to get a confirmation answer:
- Phnom Penh is the capital of Cambodia, isn't it?
We can also use a question tag when we simply want to keep the conversation going:
- It's a nice day today, isn't it?
- Oh, yes, it is.

RULES

1- A question tag has two words: the verb and the subject (+ ?)
- You can play the piano, can't you?
- She won't come here, will she?

2- If the sentence is affirmative, the question tag will be negative and vice versa.
- You are Paul, aren't you?
- You're not Paul, are you?

3- The verb, if negative, is always contracted, the subject is always a pronoun.
- Mary's been to London, hasn't she?
- Your friends don't live near here, do they?
There is no contraction for the form "am I not", so we use "aren't I".
- I'm your best friend, aren't I?

4- Only special verbs (modals and auxiliaries) can make a question tag.
- You'll help me, won't you?
- Peter may be late, mayn't he?
- Your brother's stupid, isn't he?
- You can't drive a car, can you?
So if the verb in the sentence is a normal verb, we use DO to make the question tag.
- You like pizza, don't you?
- Kevin lives over there, doesn't he?
- Your brother played basketball at school, didn't he?
- They love children, don't they?
- You feel sad, do you?

Imperative forms

For the imperative form, we use the modal verb WILL.
- Sit down, will you?
- Open the window, will you?
- Be quiet, will you?
This is not the auxiliary verb to make the future, but the modal verb that means "want".
- Come here, will you? = Will you come here? = Do you want to come here?
So using the question tag WILL YOU? with imperatives we make it sound more polite and soft. We don't need an answer here, but a nice answer is: "Yes, of course" or "Sure".

COMMON MISTAKE

In many languages, the equivalent of the English question tags is "no?", so students of those languages usually say something like this:
- You're French, no?
- He can play tennis, no?
Please, don't do this in English, it sounds funny and veeeery foreign (but they will understand)

COLLOQUIAL TAG

So, if you think question tags are not difficult, but then when you speak you can never get your tags correct, here's a very simple option: use "RIGHT?" always!

- You're French, right?
- James can speak Italian, right?
- Susan will come tomorrow, right?
- She's not very nice, right?
- You must wait here, right?
- Your family needs a new car, right?

This is very simple, and very common, especially in America. Many native speakers use this simple tag all the time, so if you use it you won't have problems, and you will sound perfect! Now you don't have an excuse to avoid question tags.