Breakdown of Он молчит, хотя всё понимает.
он
he / it (for a masculine noun) / it (for a neuter noun)
понимать
to understand
весь
all, the whole, everything, everyone
хотя
although
Questions & Answers about Он молчит, хотя всё понимает.
Do I always need a comma before хотя?
Yes! In Russian, you must always put a comma between two clauses (parts of a sentence that each have their own verb). Since молчит and понимает are two separate verbs in different parts of the sentence, the comma before хотя is strictly required.
Could this sentence start with хотя instead?
Definitely. Just like in English, you can flip the order of the parts: Хотя он всё понимает, он молчит (Although he understands everything, he stays silent).
Why is it всё here and not все?
Всё (pronounced with a yo sound) means everything and is neuter singular. Все (pronounced with a ye sound) means everyone or all and is plural. Here, he understands everything, so we need всё.
Why does он молчит translate to he stays silent rather than he is shutting up?
The verb молчать describes an ongoing state of not speaking, which is why it translates well to stays silent or is silent. If you wanted to talk about the sudden action of shutting up or becoming silent, you would use a different verb, like замолчать.