Learn Russian: Режиссёр рассердился, так как ему не понравился спектакль. - The director got angry, since he didn't like the play.

The root verb is сердить (to anger or annoy someone). Adding the reflexive postfix -ся makes the action happen to the subject (сердиться, meaning to be angry). The prefix рас-, which is a variant of раз-, adds the idea of a developing intensity or suddenly entering a state. Together, they form a perfective verb meaning to get very angry or to lose one's temper.

Why does the sentence use the dative pronoun ему instead of the nominative он?

This is due to the verb понравился (liked). In Russian, the thing being liked is the grammatical subject, which is спектакль in the nominative case. The person who likes it acts as the experiencer and must be in the dative case (ему). Literally, it translates to 'the play was not pleasing to him.'

Why is понравился in the masculine form if the sentence doesn't specify the director's gender?

The verb понравился agrees with its grammatical subject, which is спектакль (a masculine noun). Because the play is doing the action of 'pleasing,' the verb takes the masculine singular past tense form to match the play, completely independent of the director.

What is the difference between спектакль and пьеса, since both translate to 'play'?