Is it okay to say To mix work and study? Or I should say To combine work and study? As Elijah says, "combine" has a sense of mixed togther, at the same time, one result. . This can work, if it is obvious when you are talking about. . Any week in my life is a mixture of work and study. Any week in my life is a combination of work and study., . For this reason, "mix" sounds better to me too. . Mixing advice from a university teacher and from random nobodies from random parts of the world may be a recipe for fireworks. Arguing may not be good for you. :) If he/she's marking, use "combine". Being right is not as good as getting a high mark. . If the university is in Britain, maybe you also need British answers. Such collocations are frequently locale-dependent.

Either works. "Mix" sounds more informal to me.

Guys, I asked this question cuz my teacher in Russian uni said that it's not right to say mix(wrong word)

I think "mix" would be better than "combine". Combine sort of implies that they would be happening at the same time.

Both are correct!