Learn Hardcore Russian: Я стираю полотенце после купания.

In Russian, стирать means “to wash textiles” (clothes, towels) – usually with water and detergent, by hand or machine. Мыть is used for washing hard surfaces, dishes, or the body. So you стираете полотенце, а не моете его. Also, стираю is the imperfective aspect: it stresses the ongoing or habitual process, whereas the perfective постираю would stress completion.

What case is полотенце, and why doesn’t its form change?

Полотенце is in the accusative case as the direct object of стираю. Neuter nouns ending in -е have identical nominative and accusative singular forms, so полотенце stays the same.

Why is купания in the genitive case?

The preposition после requires the genitive case. Here купание (the act of bathing) becomes купания in genitive singular to mean “after bathing.”

What part of speech is купания, and why isn’t a verb used there?

Купания is a noun of action (a deverbal noun) derived from the verb купаться. Russian often uses these nouns after prepositions for conciseness. You could use a clause with a verb instead (see next question), but после купания is more compact.

How would I say “after I bathe” using a full clause instead of после купания?

You can use после того как plus a verb clause. For example:

«После того как я купаюсь, я стираю полотенце.» (imperfective, ongoing habit)
«После того как я искупался, я стираю полотенце.» (perfective, once completed)

Can I rearrange the word order and say После купания я стираю полотенце?

Yes. Russian has flexible word order. Placing после купания at the beginning simply emphasizes the time. The meaning stays the same.

What’s the difference between стираю (imperfective) and the perfective постираю or выстираю?

стираю (imperfective) describes an ongoing action or a habitual process (“I’m washing” / “I wash regularly”).
постираю or выстираю (perfective) indicates a completed action (“I will wash it (to completion)” or “I have washed it”). Use the perfective if you want to stress that the washing will (or did) finish successfully.

Could I use a reflexive form like стираюсь for “I wash myself”? It looks similar.

No. Стираюсь does not exist with the meaning “to wash myself.” Reflexive -ся on стирать would incorrectly suggest “to make oneself be washed.” To say “I wash myself,” Russian uses мыться (e.g., я моюсь or я купаюсь).