Breakdown of У меня болят мышцы, потому что я занимался спортом.
я
I
потому что
because
спорт
sport
у
at, by
Questions & Answers about У меня болят мышцы, потому что я занимался спортом.
Wait, so in У меня болят мышцы, "I" am not the subject of the sentence?
Exactly! In Russian, you don't say "I have aching muscles" using "I" as the subject. Instead, the literal translation is closer to "By me, the muscles ache." The body part (мышцы) is the actual subject in the Nominative case, and the person experiencing the pain is in the Genitive case after У (меня).
Why is the verb болят used here instead of болит?
Because the subject of the sentence is мышцы (muscles), which is plural, so the verb must also be in the plural form (болят). If only one thing hurt, like your head (голова), you would use the singular form болит.
Why does спорт change to спортом in the second half of the sentence?
The verb заниматься (to be engaged in, to practice) always requires the noun that follows it to be in the Instrumental case. To say "to do sports" or "to work out," you literally say "to engage oneself with sports," which is why спорт takes the masculine Instrumental ending -ом.