Question about German pronoun?

I'm (re-) learning German from Duolingo, after some 30 years off since high school. Today the sentence came up "Es ist ein Apfel." My question is why you would use 'Es' instead of 'Er'. After all, 'Apfel' is masculine -- der Apfel. I'm sure there's a good reason;... show more I'm (re-) learning German from Duolingo, after some 30 years off since high school.

Today the sentence came up "Es ist ein Apfel." My question is why you would use 'Es' instead of 'Er'. After all, 'Apfel' is masculine -- der Apfel.

I'm sure there's a good reason; I'd like to know what that is. Thanks!
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