Learn Russian: Он чувствовал себя подавленным, потому что скрывал внутренний гнев. - He felt depressed because he hid his inner anger.

When using the verb чувствовать себя (to feel), the adjective describing your state is usually put in the instrumental case. It acts as a descriptor of the condition you are in (literally: "he felt himself as a depressed person"). While you might hear the nominative in very casual speech, the instrumental is the standard and correct form here.

Could we use the perfective verb скрыл instead of the imperfective скрывал here?

No, скрывал is much better here because he was in a continuous state of hiding his anger. Using the perfective скрыл would imply a single, completed action (he hid it once and was done), whereas the sentence describes an ongoing background situation that caused his depression.

Is подавленный a normal adjective or something else?

It is originally a past passive participle formed from the verb подавить (to suppress, to crush), so it literally means "suppressed." However, it has fully transitioned into being used as a standard adjective meaning "depressed" or "downcast," which is how it functions in this sentence.

Why use гнев for anger instead of a more common word like злость?