"In September I sing a lot."
Translation:I september sjunger jag mycket.
It's called V2 – verb second. In any Swedish main clause that is not a question, the verb goes in second place. So if you start a sentence with a time adverbial – like i september – the verb must go right after that.
I wrote more about word order here: https://www.duolingo.com/comment/8970470
Thank you. I went to read your writing about word order. I do not know the word "ska," or I have forgotten it. For some of the examples, my lack of vocabulary makes the explanations opaque. Thank you for your efforts. I will come back to this when I get more advanced so that I can take advantage of your work.
For eftermiddag we have på eftermiddagen, i eftermiddag and i eftermiddags, with different meanings, see more here: https://www.duolingo.com/comment/6069004 I don't think I've heard på ögonblick, that would have to be på ett ögonblick, på ögonblicket, but there are other prepositions that are also possible with ögonblick.
Remember the V2 rule, in which the verb must come in the 2nd grammatical "place" in the sentence (not necessarily the 2nd word, mind you). That 2nd grammatical "place" comes after "I september" in this case, so the correct answer here would be "I september sjunger jag mycket" instead of a word-for-word English translation of "I september jag sjunger mycket"
It's a question of focus, about what it is that you're trying to communicate. If you're speaking about what you tend to do in September, it's best to start out with I september, otherwise start with the subject. We're a little bit more likely to front time adverbials in Swedish than you are in English, so that you're a tad more likely to hear I september sjunger jag mycket in Swedish than the corresponding word order in English. but the difference isn't huge. Jag sjunger mycket i september is a more neutral word order in Swedish too.