A question tag is a short question added on to a statement.  When a tag is spoken, the voice can go down or up.  We often use a question tag at the end of a sentence when we ask for agreement. 

Sometimes it is NOT a real question.  Then the voice goes down. 
If we ask a real question (because we are not sure), the voice goes up. 

FALLING
     My dress is too short, isn’t it?
     It’s a lovely day, isn’t it?    
With a falling intonation, the speaker thinks the statement is TRUE.  The tag is NOT really a question.

RISING                                      
     You went to the party yesterday, didn’t you?
     You haven’t heard a forecast, have you?
With a rising intonation, the speaker is LESS SURE.  The tag is more like a real question.     

If the statement is affirmative, the question tag is negative.  
     Roses are beautiful, aren’t they?

If the statement is negative, the question tag is affirmative.
     She isn’t naughty, is she?                                               

The question tags always end with subject pronoun (I, you, he, she, it, we, or they). If the statement verb is be, have, do, can, must, should, etc., we repeat it in the question tag.  With other verbs we use a form of do in question tag.
     My new dress looks nice, doesn’t it?
     You didn’t eat those strawberries, did you?