"I swim on Friday."
I think úszok should be accepted anyway. I was under the impression that the rules were changed about a year ago that even adding -k in 1. person singular instead of -m in -ik verbs is grammatically correct.
Some grammar rules were indeed changed a year ago, but I don't think -ik verbs were affected. It's a fact though that many people conjugate -ik verbs as regular ones. Just a few days ago I said "lakok" instead of "lakom" (but I was horrified after realizing it).
Now I am really confused. In the 1st lesson on verbs, we can find the following sentence: Gyakran úszok egyedül.
Yeah, -ik verbs are a tricky topic. I found some natives arguing about this exact verb in the reverse tree. Anyway, both "úszok" and "úszom" seem to be correct, there's no strong preference for any of the two forms, unlike in the case of "lakom" and "eszem", for example.
Do you mean that the grammar for the whole Hungarian language was changed, or just on this site?
It's fine, just the emphasis changes: I'm swimming on Friday. (After I've been running on Wednesday and cycling on Thursday.)
Also please watch your capitalisation: "Úszom pénteken."