My questions is in regards to the following sentence:
「一生懸命泳いでいる間はいやなこともわすれてしまう」
This translates to, “When I swim with all my might, I can forget about unpleasant things.” I understand that 「-しまう」denotes something is finished, but why was it used in this sentence? In what contexts should 「-しまう」be used? Or, if my translation is inaccurate, what does this sentence really say?
It can also have the sense of “end up” or (in more casual English) “wind up” as in reaching a conclusion. As in, “I end up forgetting” or “I wind up forgetting”.
Depending on context, and usually in past tense, this can have the feeling of an unfortunate result. In your sentence, forgetting unpleasant things is good, and so this likely means “I completely forget.” But in another context, わすれてしまった could mean, “Unfortunately, I ended up forgetting,” or “Unfortunately, I forgot,” as in, “I wanted to go to the party yesterday, but unfortunately I forgot.”