Learn Polish: Ten sweter jest za mały. - This jumper is too small.

When za is placed directly in front of an adjective, it acts as an adverb meaning "too" or "excessively". You will see this a lot in everyday phrases like za drogi (too expensive) or za późno (too late).

Could I say to sweter jest za mały instead of ten sweter?

No, because here "this" is acting as an adjective describing the jumper, so it must match its gender. Since sweter is a masculine noun, you must use the masculine form ten. You only use to when pointing something out generically, like in the sentence To jest mały sweter (This is a small jumper).

How would I change this sentence to say "these jumpers are too small"?

You would say Te swetry są za małe. Everything changes to the non-masculine-personal plural form. Notice that the second 'e' in sweter disappears when it becomes plural (swetry), which is a common pattern for Polish nouns ending in -er.

Is sweter borrowed from the English word "sweater"?

Yes, it is! But watch out for the spelling and pronunciation. In Polish, it is spelled phonetically with just one 'e' in the middle, and the 'w' is pronounced like the English letter 'v'.