Sorry for an awkward question, but do we say 21 student was sitting in the classroom or 21 students were sitting in the classroom (I'm not talking about ordinal numerals). I just got really confused, probably, I've overlearned English today hahaha.

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21 students were sitting in the classroom since 21 is more than 1, hence it is plural. :) You could also change 21 students into "they", that's how you check if your subject is plural or not.

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The second one.  No worries, just take a break for the day and continue tomorrow when you're refreshed :)

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21 students were siting inside of the classroom.

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The second one "21students" is correct.

Don't worry about it, take a break and start over in couple of hours.

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The correct form is 21 students were since the subject is plural.