Learn Hardcore Russian: Я купаюсь в бассейне.

The -ся suffix marks a reflexive verb. It originally means the action is performed on oneself. In купаться, it literally means “to bathe oneself,” but in everyday Russian you also use it to say “to take a swim” or “to bathe/swim.”

What case is бассейне, and why is it not accusative?

бассейне is in the prepositional case (singular). With в + prepositional you indicate location (“in the pool”). If you wanted to express motion “into the pool,” you would use в бассейн (accusative).

How is купаться conjugated in the present tense?

Here’s the full present-tense paradigm of купаться:
• я купаюсь
• ты купаешься
• он/она купается
• мы купаемся
• вы купаетесь
• они купаются

What’s the difference between купаться and плавать?

• купаться – to bathe oneself or to swim for fun/cleaning (focus on the reflexive “bathing”).
• плавать – to swim (focus on propelling yourself through water, as a sport or ability).
If you mean “I’m doing laps in the pool,” say я плаваю в бассейне. If you mean “I’m having a swim/taking a bath in the pool,” я купаюсь в бассейне is more natural.

Can I drop я and just say Купаюсь в бассейне?

Yes. Russian speakers often omit subject pronouns because the verb ending already tells you who’s doing the action. Купаюсь в бассейне clearly means “I am swimming in the pool.”

How would I say “I will swim in the pool” or “I’m going to swim in the pool”?

You have two main options:
1) Imperfective future (ongoing action): я буду купаться в бассейне – “I will be swimming/bathing in the pool.”
2) Perfective (one complete swim): я искупаюсь в бассейне – “I will take a swim in the pool.”

Where is the stress in бассейне, and how is it pronounced?

The stress falls on the second syllable: ба-ссЕ́й-не. In IPA it’s approximately [bɐˈsʲejnʲɪ].