A sentence having one principal clause and one or more subordinate clauses is called a complex sentence.

Example:

I know        +       that he will come.

P.C                                 S.C

He believes that you are a physician.

   P.C                               S.C

Now we see the question tag in a complex sentence:

Rule1: If you want to make a tag question on a complex sentence – generally the tag question is formed on the principal clause/main clause.

That is, the tag question is not formed on the Subordinate-clause.

 

 Example:

Unless you come here, I will not go. _________?

 Subordinate clause        Principal Clause

Unless you come here I will not go, will you?

Wait here, until I come back, ____? 

Principal Clause — Subordinate clause

Wait here, until I come back, will you? 

Similarly,

When he went at home, he had known everything, hadn’t he?

Rule2: If the subject of the main clause is the first person ‘I’ and the following verb is hope/believe/think/know/wish/suppose, then the question tag is formed on the subordinate clause.

 Example:

I know he is a talent man, isn’t he?

I wish that John will do well in the exam, won’t John?

I believe that Rimy will never do harm to others, will Rimy?

Rule 3: If there is no conjunction in the complex sentence, you have to form tag question on the second clause.

Example:

I think you had better leave this place, hadn’t you?   

I hope you will do better in life, won’t you?

I know you have done this mistake, haven’t you?