Learn Hardcore Japanese: kyou ha muri wo sinaide kudasai.

は after 今日 marks it as the topic: “as for today.” It can also sound slightly contrastive, like “at least today” or “today, specifically.”
Alternatives:
- 今日、無理をしないでください。 (no particle) — neutral “Today, please don’t…”
- 今日は無理をしないでください。 — topical/contrastive “As for today, please don’t…”
Don’t use 今日に here; に would mark a point in time for an action that occurs at/on that time, but topicalizing with は or just using 今日、 is natural in this kind of request.

Why is 無理 followed by を? Can I say 無理しないでください without を?

無理をする is a set phrase meaning “to overdo it / push yourself.” Here, を marks 無理 as the object of する.
Both forms are common:
- 無理をしないでください (slightly more formal/literary)
- 無理しないでください (a bit more colloquial)
Meaning is the same in everyday use.

What exactly is しないで doing here? How is it different from しなくて?

しないで is the negative て-form (ないで-form) of する. With ください, it makes a polite negative request: “please don’t do [X].”
しなくてください is generally incorrect for requests.
Contrast:
- 〜ないで: “without doing / don’t do” (for linking or requests)
- 〜なくて: often “and not/so not” or used for reasons/conditions (e.g., 困らなくてよかった “I’m glad I didn’t have trouble”), not for “please don’t.”

What does ください add? How polite is it, and can it be written 下さい?

ください makes it a polite request (“please”). It comes from the honorific verb くださる.
Politeness:
- Neutral polite: 〜ないでください
- Softer/formal: 〜ないでいただけますか, ご遠慮ください, ご無理のないようお願いいたします
- Rough/casual: 〜ないで, 〜ないでくれ, 〜するな
Spelling: As an auxiliary (“please [do]”), it’s usually written in hiragana: ください. The kanji 下さい is more for the literal “please give me [something]” sense, though many modern style guides prefer hiragana in both cases.

Who is the subject? Where is “you”?

Japanese often omits subjects and pronouns when they’re clear from context. A bare imperative/request with ください is naturally addressed to the listener, so “you” is implied.

Is 無理 a noun or an adjective? Does it mean “impossible” here?

無理 is both a noun and a na-adjective. Meanings include “unreasonable,” “impossible,” and in the set phrase 無理をする, “to overdo it / push oneself.”
In this sentence, it’s the set phrase sense: “don’t push yourself (don’t overdo it),” not “don’t do the impossible.”

How does the tone change with different variants?

Polite and neutral: 無理をしないでください。
Softer/friendly: 無理しないでね。, 無理しないでくださいね。
Strong/rough: 無理するな。, 無理はするなよ。
Very polite/deferential: ご無理なさらないでください。, ご無理のないようお願いいたします。

Can I use は after 無理 instead of を (無理はしないでください)?

Yes. 無理はしないでください puts contrastive focus on “overdoing it,” roughly “At least, please don’t overdo it.” It’s a common, slightly emphatic phrasing. With を, it’s the straightforward object of する; with は, you’re highlighting that specific aspect.

Can I add sentence-ending particles like ね or よ?

ね softens and seeks agreement/empathy: 無理しないでくださいね。 (“Okay? Please don’t overdo it, alright?”)
よ adds emphasis or warning: 無理しないでくださいよ。 (stronger, “I mean it—don’t overdo it.”)
Use based on relationship and tone you want.

Is しないで always a request? Can it also mean “without doing”?

It does both:

With ください (or on its own in context), it’s a negative request: ドアを開けないでください (“Please don’t open the door.”)
As a connector, it means “without doing”: 朝ごはんを食べないで学校に行った (“I went to school without eating breakfast.”)

What’s the nuance difference between 今日は and just 今日 at the start?

今日は…: topical/contrastive — “as for today,” often implying “at least today (even if other days might be different).”
今日、…: neutral time setting — “today,” with no particular contrast implied. Both are natural here; pick based on whether you want that contrastive feel.

Is there an even more formal or business-appropriate way to say this?

Common business/formal alternatives:

ご無理のないようお願いいたします。 (Set phrase: “Please don’t overexert yourself.”)
ご無理なさらないようお願い申し上げます。 (Very formal)
Addressing a superior directly: ご無理なさらないでください。

Why is the particle は pronounced “wa” here?

The topic particle は is spelled with the kana は but pronounced “wa.” This is a historical quirk of Japanese orthography and applies only to the particle usage (not to the syllable in regular words).

Pronunciation/reading tips for the words here?

今日 is read きょう (not こんにち in this meaning). In Tokyo pitch accent it’s typically accented on the first mora (きょ↓う).
無理 is むり.
ください is ください (stress stays even; it’s not “kudasaii”).

Why are there spaces between the words?

In normal Japanese writing, spaces are not used between words. They’re added in learning materials to show word boundaries and particles. The sentence would usually be written as: 今日は無理をしないでください。