Somebody told me that it is prohibited to say the prayer behind any person who commits shirk (polytheism). But, as you know, saying prayer in the mosque is recommended. Now the imam of our mosque is a person who goes to the graves of saints and invokes them for help, which, of course, is shirk. But my problem is that there is no other mosque near my house. What should I do in this situation?

Subscribing to polytheism, which means willfully adopting it, is one thing and succumbing to a form of polytheism while at the same time thinking that it is not polytheism is entirely another. While the former makes a person a grave sinner in the eyes of the Almighty, the latter may afford him some excuse if he has fallen into it in spite of sincerely trying to avoid it. No doubt, people who invoke the help of saints are committing a form of polytheism, but if you have a discussion with them on this, they will tell you that what they do is certainly not polytheism and falls within the ambit of monotheism.

Praying behind ‘such’ imams cannot be regarded as prohibited. Of course, if you had a choice you could have gone to some other mosque. Since you do not, praying in congregation behind an imam who has some wrong views is better than praying individually at home.