Learn Dutch: Zwem je niet? - Do you not swim?

Normally, the verb for je (you) gets a -t at the end (like Je zwemt). But when je or jij comes immediately after the verb in Dutch, that -t is dropped. This rule kicks in all the time for yes/no questions!

Why does niet come at the very end of this question?

When you want to negate the verb itself (meaning you are saying the action of swimming is not happening), niet usually goes at the end of a simple sentence. Because this is a question, the verb (Zwem) and subject (je) are at the front, and niet simply takes the final spot.

Can this also mean "Are you not swimming?"

Yes! Dutch doesn't have a direct equivalent to the English "-ing" form (present continuous) for everyday use. Zwem je niet? covers both a general habit ("Do you not swim?") and an action happening right now ("Are you not swimming?").

Could I say Zwem jij niet? instead?

Yes, you can. je is the unstressed, everyday word for "you". You would use jij if you want to specifically emphasize the person, like if you're asking: "I am going in the water, are you not swimming?"