Questions about Jouer
Q: Why is it ma soeur joue du piano and NOT ma soeur joue le piano?
When you mean "play an instrument", use jouer de; when you mean "play a game/sport", use jouer à.
Q: Why is it j'aime jouer au baseball and NOT j'aime jouer à baseball?
It's true that jouer is followed by à if you are writing about a sport or a game. However, French nouns are usually preceded by le or la. Since the combination à + le = au, you need to write jouer au baseball.
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