Hello.

I will be very grateful for your help.
1)Do you think it's possible to say: "at the end of the 19th - the beginning of the 20th centuries (maybe,century?)" or do I have to use the word "century" twice?
2)Do you think the sentence sounds OK: "It is the relationship between the two parties, their role in the development of the country that are the subject of analysis in the presented material" (maybe, "the presented paper" - it's a report)?
3)Do you think it's possible to say: "the author has examined a large number of works of researchers of the Soviet times from 1975 till 1985" (I'm especially interested in the word "works", maybe, "publications", "research papers", "researches")?
4)Do you thik it's normal to say: "foreign and domestic scholars", I mean "domestic" are those who live and work in my country, maybe, "national"?
5)Which word is better in the context: "political developments taking place/happening/occurring in the Soviet Union"?
6)Do you think it's OK to say: "The USA insisted on... while the USSR preferred... , a difference which complicated their relationship. (I mean, will the sentence be clear and do I need a comma before "difference")"
Thank you very, very much for your help.

1)Do you think it's possible to say: "at the end of the 19th - the beginning of the 20th centuries (maybe,century?)" or do I have to use the word "century" twice?

Better would be this: at the end of the 19th or the beginning of the 20th century ...

2)Do you think the sentence sounds OK: "It is the relationship between the two parties, their role in the development of the country that are the subject of analysis in the presented material" (maybe, "the presented paper" - it's a report)?
Better would be this: It is the relationship between the two parties and their role in the development of the country that are ...

3)Do you think it's possible to say: "the author has examined a large number of works of researchers of the Soviet times from 1975 till 1985" (I'm especially interested in the word "works", maybe, "publications", "research papers", "researches")?
"publications" would definitely be the word to use in that place, yes.

4)Do you thik it's normal to say: "foreign and domestic scholars", I mean "domestic" are those who live and work in my country, maybe, "national"?
"national" would be better, yes.

5)Which word is better in the context: "political developments taking place/happening/occurring in the Soviet Union"?
I'd use "occurring."

6)Do you think it's OK to say: "The USA insisted on... while the USSR preferred... , a difference which complicated their relationship. (I mean, will the sentence be clear and do I need a comma before "difference")"
You need to keep the comma before "a difference" AND you also need to add a comma before "while" since the "while" clause is a parenthetical clause. See #4 here:
" target="_blank">http://grammar.ccc.commnet.edu/grammar/commas.htm

1) When referring to a time period that spans across two centuries, it is correct to use the word "century" only once. So, you can say "at the end of the 19th - beginning of the 20th century."

2) The sentence "It is the relationship between the two parties, their role in the development of the country that are the subject of analysis in the presented material" is grammatically correct. However, depending on the context, you may consider using "the presented paper" instead of "the presented material" if it is more appropriate.

3) The sentence "the author has examined a large number of works of researchers of the Soviet times from 1975 till 1985" is understandable, but it can be improved. Instead of "works," you could use "publications" or "research papers" to be more specific. For example, "the author has examined a large number of publications/research papers by researchers from the Soviet times between 1975 and 1985."

4) The phrase "foreign and domestic scholars" is commonly used, so you can use it. However, if you prefer an alternative, you could use "national scholars" to refer to those who live and work in your country.

5) In the context of political developments in the Soviet Union, you can use any of the three options: "political developments taking place in the Soviet Union," "political developments happening in the Soviet Union," or "political developments occurring in the Soviet Union." All three are grammatically correct, and the choice depends on your preference.

6) The sentence "The USA insisted on... while the USSR preferred..., a difference which complicated their relationship" is clear and understandable. However, it would be better to include a comma before "a difference," like this: "The USA insisted on... while the USSR preferred..., a difference which complicated their relationship." This comma helps to separate the introductory phrase "a difference" from the rest of the sentence.