Learn Hardcore Japanese: supootu wo sita ato, kimotiyoku syawaa wo abite nemasu. - After doing sports, I take a refreshing shower and go to sleep.

About the tense: した is past form, but in this pattern it does not mean the whole sentence is in the past. It just shows that one action is completed before the next one. The overall time (past / present / future / habitual) is decided by the last main verb 寝ます.

So:

スポーツをしたあと、気持ちよくシャワーを浴びて寝ます。
= Whenever/after I play sports, I then shower pleasantly and sleep.

Even in polite speech, this part stays in plain past form. You do not say スポーツをしましたあと.

What is the difference between あと, あとで, and 〜てから here? Could I say スポーツをしたあとで or スポーツをしてから instead?

All three are possible; the differences are small:

Verb-たあと(で)
- スポーツをしたあと、シャワーを浴びて寝ます。
  Neutral after doing X. あと and あとで are both fine here:
- スポーツをしたあと、…
- スポーツをしたあとで、…
  あとで can sound a tiny bit more like “at some later time after…”, but in many cases they are interchangeable.
Verb-てから
- スポーツをしてから、シャワーを浴びて寝ます。
  Also means after doing X, often with a feeling of “do X first, and then (as the next step) do Y”. It emphasizes the order more like step 1 → step 2.

In this sentence, all of these are natural:

スポーツをしたあと、気持ちよくシャワーを浴びて寝ます。
スポーツをしたあとで、気持ちよくシャワーを浴びて寝ます。
スポーツをしてから、気持ちよくシャワーを浴びて寝ます。

The nuance difference is small; a learner can treat them all as after doing sports here.

What exactly does 気持ちよく mean, and why does it end in く instead of い?

The base adjective is:

気持ちいい (or 気持ちがいい) = feels good / pleasant / comfortable

To make an adverb (like pleasantly, nicely), Japanese usually changes an い-adjective:

〜い → 〜く

Examples:

早い (fast) → 早く (quickly)
楽しい (fun) → 楽しく (enjoyably)

So:

気持ちいい → 気持ちよく

気持ちよくシャワーを浴びて寝ます literally means:

“I take a shower in a way that feels good, and then sleep.”

Natural English would be something like:

“I take a nice/refreshing shower and go to sleep.”

気持ちよく is describing how you shower (pleasantly), not what the shower is. If you wanted to say “a pleasant shower” as a noun phrase, you’d use:

気持ちのいいシャワー (a pleasant-feeling shower)

Why is there no subject like 私 in the sentence? How do we know it means “I”?

Japanese often omits the subject when it’s obvious from context.

This sentence is talking about a personal routine:

スポーツをしたあと、気持ちよくシャワーを浴びて寝ます。

In a normal conversation about your habits, listeners will automatically assume I unless there is a reason to think otherwise.

If you really want to make it explicit, you can say:

私はスポーツをしたあと、気持ちよくシャワーを浴びて寝ます。

To say he or she, you’d use:

彼はスポーツをしたあと、… (he)
彼女はスポーツをしたあと、… (she)

But in natural Japanese, these pronouns are only added when needed to avoid confusion.

Why do you say シャワーを浴びる for “take a shower”? What does 浴びる mean?

The verb 浴びる means to bathe in / to be showered with / to be exposed to something (like water, light, etc.).

Common patterns:

シャワーを浴びる = to take a shower
日光を浴びる = to be exposed to sunlight
批判を浴びる = to come under criticism (figurative)

So シャワーを浴びて寝ます literally is:

“I bathe in a shower and then sleep.”

Natural English: I take a shower and go to bed.

You might hear casual シャワーする in speech, but the standard expression taught and used in writing is シャワーを浴びる.

What is the function of the て-form in 浴びて寝ます?

The て-form of a verb is often used to connect actions:

Verbて、Verbます
= do A and (then) do B

In this sentence:

シャワーを浴びて寝ます。
= “I take a shower and (then) sleep.”

Key points about て-form here:

It shows a sequence: shower → sleep.
It implies the same subject is doing both actions.
The tense/politeness of the whole sequence is decided by the last verb (寝ます).

So:

スポーツをしたあと、気持ちよくシャワーを浴びて寝ます。
has one main tense (non-past polite) given by 寝ます; 浴びて just links the action before it.

What tense or time does 寝ます express here? Does it mean “I sleep”, “I will sleep”, or “I usually sleep”?

寝ます is the non-past polite form of 寝る.
Non-past in Japanese can cover:

future: “will sleep”
habit/routine: “(usually) sleep”
sometimes general present: “sleep” (as a general fact)

Which meaning you get depends on context.

In this sentence, it can reasonably be understood as:

A habit/routine:
- “After I play sports, I take a nice shower and go to bed.”
Or a future plan, if used about a specific upcoming time:
- “After I play sports, I’ll take a nice shower and go to sleep.”

Japanese does not force a strict future vs present distinction here; the context decides.

Why is 寝ます in polite form? How would the casual version of this sentence look?

寝ます is the polite -ます form of 寝る.
The sentence as given is in a polite style suitable for talking to someone you’re not very close to, or in writing:

スポーツをしたあと、気持ちよくシャワーを浴びて寝ます。

To make it casual/plain, you generally:

Use plain forms instead of -ます forms, and
Often simplify or drop some particles.

A natural casual version might be:

スポーツしたあと、気持ちよくシャワー浴びて寝る。

Notes:

したあと is already in plain form, so it stays the same.
浴びて (て-form) is the same for polite and plain; only the final verb changes to show politeness.
Dropping を before した and 浴びて is common in casual speech, but not required.

Can I drop the particle を in parts of this sentence, like スポーツしたあと or シャワー浴びて寝ます?

Yes, in casual spoken Japanese, dropping を is common in some cases, but you should know when it’s safe.

With 〜する verbs (like スポーツする, 勉強する), を is often optional:
- Polite/clear: スポーツをしたあと
- Casual: スポーツしたあと

Both are natural.

With シャワーを浴びる, in standard Japanese you keep を:
- Standard: シャワーを浴びて寝ます。

In very casual speech you may hear:

シャワー浴びて寝る。

This is okay in relaxed conversation, but for learners (especially in writing or polite speech), it’s safer to keep the particle:

スポーツをしたあと、気持ちよくシャワーを浴びて寝ます。

Why is あと written in hiragana instead of the kanji 後?

The word あと (meaning “after / later”) can be written as:

あと (hiragana), or
後 (kanji), or
mixed forms in some compounds (その後, 後で, etc.)

For very common everyday words, Japanese often prefers hiragana in ordinary writing, especially in:

textbooks
children’s materials
places where readability is prioritized

So:

スポーツをしたあと、…
and
スポーツをした後、…

are both correct. Using あと in hiragana simply makes the sentence a bit lighter and easier to read.

Is the comma after あと necessary? How does punctuation work with this kind of “after …, …” structure?

The comma (、) after あと is not strictly required, but it is:

very common, and
recommended for clarity.

So:

スポーツをしたあと、気持ちよくシャワーを浴びて寝ます。 (standard, easy to read)
スポーツをしたあと気持ちよくシャワーを浴びて寝ます。 (still grammatical, just a bit denser)

In Japanese, a phrase like スポーツをしたあと functions as an adverbial clause (“after doing sports”). It’s natural to separate that clause from the main part of the sentence with a comma, just like in English:

After I play sports, I take a nice shower and go to sleep.

So the comma is a punctuation choice, not a grammar requirement, but it makes the structure clearer.