Tag questions

When the subject in the first sentence is in plural you must use "THEY" in the question tag to substitute it. 

Examples:
My students´ aren´t at school, are they?
Are  your parents at home, aren´t they?

When in the first sentence is used the pronoun "I" with the verb to be "am" we are going to change it in the tag question by te verb "are".
Examples:
I am invited to your birthday, aren´t I?
I am free of work, aren´t I?

Remember that question tags are always in the opposite form of the main sentence. Positive with negative and negative with positive.
Examples:
She is happy, isn´t she?
He is a great doctor, isn´t he?

In simple present (not verb to be) we need to use in our question tag the auxiliar Do or Does. We will use "Do" for plural and the first and second singular person ( I-You-We-They), and, we will use "Does" when the subject is a third person (She-He-It)

Examples:
She cares about her mother, doesn´t she?
We play together today, don´t we?  

Remember the main sentence and the tag question will always have the same verbal time.
Examples:
Your father bought a new car, didn´t he?
You won´t come to visit me tomorrow, will you?

Wen in the main sentence we have a modal verb (Can-Could-Would-Should) the questin tag will have it too.
Examples:
Would you prefer strawberry ice cream, wouldn´t you?
He can´t  buy some fruit to eat, can he?