Questions & Answers about Я работал там с мая до сентября.
Why do the months end in -я here (мая, сентября)?
Both the prepositions с (meaning from a certain time) and до (meaning until) require the Genitive case. The dictionary forms are май and сентябрь. They both take the -я ending in the Genitive because they are masculine nouns ending in -й and a soft sign, respectively.
I thought the word for from was из or от. Why is it с in this sentence?
In Russian, the preposition for from changes depending on the context. When specifying the starting point of a period of time, you must use с. You would use из for moving out of an enclosed place, and от for moving away from a person or an object.
Could I use по instead of до for until September?
Yes, but it changes the exact timeframe. с мая до сентября technically means you worked up until September began (excluding September itself), though in casual speech people sometimes use it loosely. If you want to explicitly say you worked through the end of September, you would use по plus the Accusative case: по сентябрь.
Why use the imperfective verb работал instead of the perfective поработал here?
You are stating a fact about a continuous state or routine that took place over a specific duration of time. The imperfective aspect (работал) naturally pairs with this emphasis on the ongoing process of working during that span, rather than focusing on the completion of a single discrete action.