Questions & Answers about Вчера мы с братом заехали к Анне.
Why does the sentence use мы с братом instead of мой брат и я?
While мой брат и я is grammatically understood, Russian strongly prefers the construction мы с (we with) followed by the noun in the Instrumental case. So мы с братом literally means "we with brother," but it is the standard way to say "my brother and I." Notice that the verb заехали takes the plural ending because the grammatical subject is "we."
Why use заехали instead of зашли to mean "dropped in"?
Both verbs use the prefix за- to mean "stopping by" or "dropping in," but they use different verbs of motion as their base. заехали is built from ехать (to go by transport), which tells us they drove. If they had walked, the sentence would use зашли, which is built from идти (to go on foot).
Why is Anna in the Dative case (к Анне)?
When you drop in to see a person, Russian requires the preposition к followed by the Dative case. If you were dropping in at a place rather than visiting a person, you would use в or на followed by the Accusative case instead (for example, заехали в магазин for "dropped into the store").
Does the verb заехали imply anything about how long they stayed with Anna?