"Yüzmek çok güzel. It's very nice to swim."

Hi bod. There is nothing wrong with the sentences above and I guess I know what has confused you. Lets look at the following sentences:

1. Küçük otomobil gitti. (The small car went away)
2. Küçük bir otomobil gördüm. (I have seen a small car)
3. Bu otomobil küçük. (This car is small)

In the first and second sentence the adjective küçük modifies the noun otomobil. On the last sentence the adjective doesnt modify a noun. It is in a role of verb. We can use nouns or adjectives in the role of verbs:

4. Sen akıllısın. (You are clever.)
5. Ben doktorum. (I'm a doctor.)

These kind sentences, where a noun or adjective is used as a verb, like in 3, 4 and 5, can be build with "to be" in English as you see from the translations of 3,4 and 5.

Actually the following sentences have the same structure as 3,4,5

6. Bu köpek yaramaz. (This dog is naughty )
7. Yüzmek çok güzel. (It's very nice to swim)
8. Dondurma çok güzel. (This ice cream is very nice)
9. Sen çok güzelsin. (You are very beautiful)

Because the third singular doesnt take a personal suffix with the sentences constructed on a noun or adjective they are more likely to be confused.

Julia çok güzel.
Yüzmek çok güzel.
Sen çok güzelsin.

have all the same gramatical structure.