I recently came to the understanding that teachers in China tell their students that 'in' doesn't go together with 'there', for instance explaining that something is 'in there'. Teachers disagree with this, and, to be honest with you, that disagreement made me confused as I've been trying to find this situation in texts, articles and so on, and been very unfortunate to find any example. However, I know I use these two words together and never really realized that I could be wrong.

Now, my question to you all is, have I been wrong all these years and no one has ever corrected me or....?

Appreciate your suggestions for this matter!

in there is a possible combination, but it's possible that there are sentences where it is awkward.

You would have to give us some sentences.

I think it is a pretty common phrase. Say you're a wildlife biologist and you are studying wolf dens and fecundity, you may well use this group of words:

Justin - Can you see how many wolf pups there are in the lair?

Gertrude - Just about.... a little bit further.... there, I can see them now....

Justin - Well, how many pups are in there?

Gertrude - There are four pups that have been born to the alpha female.

What examples can you provide where you've used these words?

In there is a specific reference to something of interest/reference that is contained within something else. In the above example the number of wolf pups in a grotto, or lair.