Learn Russian: В реке нельзя купаться, вода очень холодная. - Swimming in the river is not allowed, the water is very cold.

The word нельзя is impersonal, meaning it describes a general rule rather than what a specific person is doing. To say 'it is not allowed to swim' or 'you must not swim', you simply combine нельзя with an infinitive verb like купаться. No subject like 'I' or 'you' is needed.

Why is the imperfective verb купаться used with нельзя here instead of a perfective verb?

When нельзя means 'it is forbidden' or 'it is strictly not allowed', it almost always pairs with imperfective verbs. Using the imperfective emphasizes that the action should not be done at all, under any circumstances.

Why does this sentence use купаться for swimming instead of плавать?

While плавать refers to the physical exercise or directional motion of swimming, купаться means to bathe, splash around, or swim for fun. When talking about swimming at a beach, lake, or river for recreation, Russians almost always use купаться.

Why is it в реке (Prepositional case) rather than в реку (Accusative case)?

We use the Prepositional case в реке because it answers the question 'where?' to describe a static location. If the sentence was about moving or jumping into the river, answering the question 'to where?', you would use the Accusative case в реку.