Translation:The orchestra was conducted by the conductor.
It is a general rule for weak German masculine nouns. They usually have endings like "-e", "-ent", "-ist" and should have a suffix "-(e)n" added at the end when used in accusative, dative and genitive. For example, "Er hat dem Presidenten einen Brief geschrieben" which means he wrote a letter to the president.
Why is vom being used for "by" here? Earlier DL said that vom means from / for, which I came to terms with even though there is a separate word "für" for "for". (Why used another word to mean what an existing word already means?) All that von does that für doesn't do is convert things to dative, which for sure increases the complexity for non natives. If someone would please let me know the difference between von and für to mean for, I shall be thankful. And there's more to come. Now, 'von' is being used to mean 'by' when there's already a word for that i.e. 'bei'. Why is this? (PS: This was a light hearted rant so don't take it seriously
Prepositions are used in different ways in different languages, so they often can't be translated one for one between any two languages. It's difficult for learners because we have to consider the context of the sentence and know or guess the correct preposition that is used with those nouns and verbs in that context. I think the only way I am going to really learn how they are used is to do lots of reading of standard German text. The Duo discussion threads are really helpful to get started though. There is so much to learn at once. Viel Glück!