The Explanation of Question Tags

Grammar explanation

We can add question tags like isn't it?, can you? or didn't they? to a statement to make it into a question. Question tags are more common in speaking than writing.

We often use question tags when we expect the listener to agree with our statement. In this case, when the statement is positive, we use a negative question tag.

She's a doctor, isn't she?
Yesterday was so much fun, wasn't it?

If the statement is negative, we use a positive question tag. 

He isn't here, is he?
The trains are never on time, are they?
Nobody has called for me, have they?

If we are sure or almost sure that the listener will confirm that our statement is correct, we say the question tag with a falling intonation. If we are a bit less sure, we say the question tag with a rising intonation.

Formation

If there is an auxiliary verb in the statement, we use it to form the question tag.

I don't need to finish this today, do I?
James is working on that, isn't he?
Your parents have retired, haven't they?
The phone didn't ring, did it?
It was raining that day, wasn't it?
Your mum hadn't met him before, had she?

Sometimes there is no auxiliary verb already in the statement. For example, when:

... the verb in the statement is present simple or past simple and is positive. Here we use don't, doesn't or didn't:

Jenni eats cheese, doesn't she?
I said that already, didn't I? 

... the verb in the statement is to be in the present simple or past simple. In this case we use to be to make the question tag:

The bus stop's over there, isn't it?
None of those customers were happy, were they?

... the verb in the statement is a modal verb. Here we use the modal verb to make the question tag:

They could hear me, couldn't they?
You won't tell anyone, will you?

If the main verb or auxiliary verb in the statement is am, the positive question tag is am I? but the negative question tag is usually aren't I?:

I'm never on time, am I?
I'm going to get an email with the details, aren't I?