Learn Hardcore Japanese: yoru ha kaaten wo simete nemasu. - At night I close the curtains and sleep.

夜に寝ます simply states “I sleep at night” (time expression).
夜は寝ます (or in our sentence 夜はカーテンを閉めて寝ます) emphasizes “When it comes to night…” or “At night (as opposed to day)… I close the curtains and sleep.”

What does the particle を do after カーテン?

を marks カーテン as the direct object of the verb 閉める (“to close”). It tells you what is being closed. If you used が with 閉まる, it would mean “The curtains close (by themselves).”

Why is 閉めて in the て-form before 寝ます?

The て-form here connects two actions in sequence or with little pause:

閉めて – “close (the curtains),”
寝ます – “then sleep.”
Together it means “I close the curtains and (then) go to sleep.”

Why is the verb 寝ます in the polite ます-form? Could I say 閉めて寝る instead?

寝ます is the polite form, suitable for neutral or polite statements.
In casual speech or writing you can indeed say 閉めて寝る (“close them and sleep”).

Where’s the subject in this sentence?

It’s omitted. Japanese often leaves out the subject when it’s obvious from context. Here it’s understood as “I” (or “we,” depending on who’s speaking).

Can I say 夜にカーテンを閉めて寝ます? What’s the nuance difference?

夜に～ focuses on the exact timing (“at night”).
夜は～ makes 夜 the topic, implying a regular or contrasted habit (“as for nights…”).
Depending on what you want to stress—time vs. topic—you choose に or は.

What’s the difference between 閉める and 閉まる?

閉める (shimeru) is transitive: “to close (something).”
閉まる (shimaru) is intransitive: “(something) closes” or “is closed.”

Why didn’t we say 閉めてから寝ます to show “after closing”?

Using the て-form alone already implies “do A, then B.”

閉めて寝ます is natural for habitual actions.
閉めてから寝ます (“after closing, I’ll sleep”) is also correct but more explicit about the order, often used when you want to emphasize the sequence.