1. The fish I ate upset me.

2. The fish I had eaten upset me.

(One more similar question, please. Are both OK? Because of the tense, #2 seems to be better. What about #1? Are both OK? Is there any difference between them?)

3.I had finished the work before my boss came back.

4. I finished the work before my boss came back.

(Between #3 and 4, which one is better? Are both OK as well?)

both of them are correct....

the difference is that one of them is in past tense and the other is in past perfect tense..
but you can use any one of them,its okay

Yes, you can use either sentence in each of the pairs, but if you are to be following the sequence of tenses strictly, then use only 2 and 3.

http://grammar.ccc.commnet.edu/grammar/verbs.htm#sequence ~~~> http://grammar.ccc.commnet.edu/grammar/sequence.htm

1. "The fish I ate upset me" and "The fish I had eaten upset me" are both grammatically correct, but they have subtle differences in meaning.

- "The fish I ate upset me" suggests that the act of eating the fish caused you to become upset in that moment. It implies a cause-and-effect relationship in the present or recent past.

- "The fish I had eaten upset me" implies that you ate the fish at some point in the past, and as a result, you were upset. It suggests that the upset feeling occurred later, after consuming the fish.

2. Similarly, "I had finished the work before my boss came back" and "I finished the work before my boss came back" are both grammatically correct, but they convey different time relationships.

- "I had finished the work before my boss came back" indicates that the completion of the work occurred in the past before your boss's return. It emphasizes the past action being completed before another past event.

- "I finished the work before my boss came back" simply states that the work was completed before your boss returned. It doesn't place as strong an emphasis on the sequence of events as the previous sentence.

In summary, both choices are grammatically correct, but the use of different tenses conveys different temporal and causal relationships between events. The choice between them depends on the specific meaning you want to convey.