Learn Russian: Я не знал, насколько он устал.

In English, you combine "how" with an adjective ("how tired"). In Russian, как usually means "in what manner." When asking "to what degree" or "how much," you must use насколько. Because Russian uses a verb for "tired" here, насколько perfectly expresses "to what extent he got tired."

Why does Russian use a past-tense verb (устал) for the English adjective "tired"?

Устать is a perfective verb meaning "to get tired." Russian frequently expresses physical states—like being tired, drunk, or frozen—using the past tense of a perfective verb. This emphasizes the result of an action. So он устал literally means "he got tired," which translates naturally to the ongoing state of "he was tired."

English says "was tired" due to tense shifting ("I did not know..."). Is that why устал is in the past tense?

No, Russian does not backshift tenses like English does. Usually, to express a simultaneous action in the past, Russian keeps the subordinate verb in the present tense (e.g., Я не знал, что он делает - I didn't know what he was doing). However, because the state of being tired is the result of a completed action, the past perfective verb is used to show he had already reached that state.

Why is there a comma before насколько?

In Russian grammar, a comma is strictly required to separate a main clause from a subordinate (dependent) clause. Here, насколько introduces the subordinate clause, so the comma before it is mandatory. English doesn't use a comma in "I didn't know how..." so this is an easy punctuation rule to forget.