I still have some sentences to check. Thank you very much to you all!

1) He was one of the most famous foreigners participating in the efforts against the fascist rebels/the fascists/fascism (are they all possible?)
2) His indifference to accepted values is caused by various episodes in his life (??).
3) His personality is characterized by an indifference to accepted values and by a strong-mindedness caused by his failing to adhere to Eton's strict moral code.
4) Eton is an independent school for boys aged 10 to 18 renowned for its use of corporal punishment ( to which boys were subjected ?).
5) They received a birching (not they were birched?) on their bare posterior during semi-public events (better during a semi-public ceremony)
He experienced poverty first hand by working as a dishwasher.
6) In 1936 he reported on the Spanish Civil War between the Fascists and the Republican government.
After his return to England he moved to London where he worked for the BBC.
7) Winston is in a prison cell because O'Brian, whom he thought was a member of the Brotherhood, has arrested (?) him and told that Julia, his lover, had betrayed him.

1) Yes. All are possible.

2) Right.

4) You don't need the parenthetical statement.

5) a semi-public

6) Right

1) Yes, all the options are possible. You can say "He was one of the most famous foreigners participating in the efforts against the fascist rebels," "He was one of the most famous foreigners participating in the efforts against the fascists," or "He was one of the most famous foreigners participating in the efforts against fascism."

2) The sentence "His indifference to accepted values is caused by various episodes in his life" seems grammatically correct. However, without more context, it is difficult to determine if the usage of "indifference to accepted values" is appropriate.

3) The sentence "His personality is characterized by an indifference to accepted values and by a strong-mindedness caused by his failing to adhere to Eton's strict moral code" is grammatically correct and clear in its meaning.

4) The sentence "Eton is an independent school for boys aged 10 to 18 renowned for its use of corporal punishment" is clear. However, the addition of "to which boys were subjected" at the end of the sentence provides extra clarification and makes the meaning more explicit.

5) The sentence "They received a birching on their bare posterior during semi-public events" is understandable. However, replacing "during semi-public events" with "during a semi-public ceremony" may make the sentence more specific and clear in its meaning. As for the phrase "not they were birched," this seems like a possible correction to make the sentence more grammatically accurate. "They were birched" would be the correct phrase to use.

6) The sentence "He experienced poverty firsthand by working as a dishwasher" is grammatically correct and clear.

7) The sentence "Winston is in a prison cell because O'Brian, whom he thought was a member of the Brotherhood, has arrested him and told him that Julia, his lover, had betrayed him" is grammatically correct. However, the phrase "arrested him" would be more appropriate to use instead of "arrested."