"Mom began to clean the room."

Translation: Mama zaczęła sprzątać pokój.

[Kasia_T]

Mama zaczęła czyścic pokój
should also be a valid answer.

[vytah]

I wouldn't say so.

Czyścić means to remove dirt/contamination from something. For example, cleaning shoes, furniture, cars and so on. If water is involved, you usually use myć, for example cleaning floor or doing dishes.

Sprzątać means to make something neat/ordered by rearranging or removing things. For example, cleaning rooms, backyards, drawers, wardrobes and so on.

There's some overlap in meanings.

Czyścić pokój would suggest removing large old stains that are all over the room.

If cleaning a room involves putting strewn things back on their places, vacuuming, and other ordinary things, it's sprzątać.

[Kasia_T]

There is nevertheless some ambiguity in the sentence, in that it simply says "clean". I understand that you may want the user to input the most commonly used definition, in this case sprzątać, czyścić is still a valid translation of 'to clean' and should also be accepted as correct given lack of further context.

[RichardWelzel]

I tried the following translation: "Mama zaczęła sprzątać w pokóju." But that was not accepted. Why?

[Jellei]

Technically it means "in the room", but those mean basically the same, so added now.

One thing though: it is "pokÓj", but "w pokOju".