Put the word "only" after the verb to be precise. When the word "only" is before the verb, it technically means "that's the one and only thing the subject of the sentence does." The word "only" limits anything the subject does to that specific verb. After the verb, the word "only" limits the verb not the subject. "This water park only opens in the summer" -- This water park does nothing else but open in the summer. "This water park opens only in the summer" - When this water park opens, it will open in the summer and no other season. -- which is obviously what is meant here. However, in casual conversation, you will hear the word "only" before the verb. When this happens, the speaker intends the meaning to be verb/only and not only/verb. You can correct them at the risk of being annoying, because you know what that person means (they don't mean that's all the water park does.) The position of "only" is more important in formal writing than in casual speech.

The restaurant opens only from 8pm to 12pm The water park opens only in the summer

The restaurant only opens from 8pm to 12pm. Is the correct sentence. But it would make more sense to say: 1) The restaurant is only open from 8pm to 12pm. 2) The restaurant is open from 8pm to 12pm.

The correct sentence are : 1. The restaurant opens only from 8 pm to 12 pm. 2. This water park opens only in the summer