"The stressed syllable in Russian is not changed in this case". Can I replace "is not changed" to "doesn't change"? Why? Could you me explain me the reasons? Yes, you can replace "is not changed" with "doesn't change". "Is not changed" implies that a speaker will not change it. For example, "The stressed syllable in Russian is not changed by the speaker in this case." "Does not change" implies that the syllable itself will "do" or "not do" the changing. Both forms are correct because both statements are true. I really hope that makes sense.

Teacher Anne, Thank you very much for your explanation. It makes sense!