Questions & Answers about Mit wem streitest du?
Why is it wem and not wer or wen?
Because the preposition mit (with) always requires the dative case. The dative form of the question word wer (who) is wem (whom). So, mit wem literally translates to "with whom".
Why is mit at the beginning of the sentence when English puts "with" at the end?
In German, prepositions must stay attached to the question word they belong to. While in English you can split them up and say "Who are you arguing with?", German strictly requires "With whom are you arguing?" (Mit wem streitest du?).
Can streiten also mean "to fight" like in a physical fight?
streiten usually refers to a verbal argument. If you want to talk about a physical fight, you would normally use a different verb like kämpfen.
Why does streiten become streitest here?
Because the subject is du (you). Normally, you just add -st for the du form. However, because the root of streiten ends in a "t" (streit-), you must add an extra "e" to make it easy to pronounce: streitest.