Learn Russian: Мой соперник был вынужден пожертвовать ферзя. - My opponent was forced to sacrifice the queen.

вынужден is a short-form adjective meaning "forced" or "compelled." Like all short adjectives, it functions as the predicate and agrees with the subject in gender and number. Because the subject соперник is masculine singular, we use the masculine past-tense linking verb был followed by the masculine short form вынужден. The phrase then links directly to the infinitive пожертвовать.

Why is the accusative form of the chess queen ферзя? Isn't a game piece inanimate?

In reality, yes, but Russian grammar treats chess pieces, playing cards, and dolls as animate nouns! Because ферзь is a masculine noun, its accusative case takes the same form as the genitive case (ферзя) rather than the nominative, exactly as it would for a living person or animal.

Could I just say должен был instead of был вынужден?

You could, but it changes the nuance. While должен был means "had to" or "was obliged to," был вынужден specifically means "was forced to" by external circumstances against one's will. Since a player rarely wants to sacrifice their most powerful piece in chess, вынужден perfectly captures that the opponent's moves left them with absolutely no other choice.

Why does вынужден take the perfective verb пожертвовать here instead of the imperfective жертвовать?