Questions & Answers about Брат не может найти сестру.
Why do we use найти (to find) instead of conjugating it?
This is the standard rule for modal verbs like мочь (to be able to/can). The main verb может is conjugated to match the subject (брат), and the second verb stays in its infinitive (dictionary) form, найти.
The dictionary form is мочь (to be able to). Why does it become может here?
The verb мочь is irregular. For he/she/it subjects, the root changes to take a 'ж' sound and an -ет ending, becoming может. You just have to memorize its forms, like я могу (I can) and он может (he can).
Why is the word for sister сестру here, and not сестра?
The sister is the direct object of the sentence, meaning she is the one being found. Because of this, we use the Accusative case. For feminine nouns ending in -а like сестра, the ending changes to -у, making it сестру.
The English translation says 'find his sister', but there is no word for 'his' in the Russian sentence. Why?
In Russian, possessive pronouns like 'his' (его) or 'my' (мой) are often left out when talking about family members or personal items, as long as it is obvious from the context whose sister it is!