Learn Hardcore Japanese: zyousi ha, buka ga byouki de nyuuinsite iru aida ha muri wo sasenai you ni minna ni tanomimasita. - The boss asked everyone not to overwork their subordinate while he is hospitalized.

上司は = As for the boss, / The boss, …
If you said 上司が, it would sound more like you’re emphasizing that it was the boss (not someone else) who did the action.

In neutral narration, using は for the main person you’re talking about is very natural:

上司は … 頼みました。
As for the boss, (he/she) asked …

Why does the sentence use が after 部下 instead of は?

部下が is marking 部下 as the subject inside a subordinate clause:

部下が病気で入院している間
the period while the subordinate is in the hospital due to illness

Here, the whole clause 部下が病気で入院している modifies 間 (time period). Inside such modifying clauses, が is the normal subject marker.

Using 部下は here would give an odd focus or contrast and is not standard in this kind of relative clause. So:

Main topic of the whole sentence: 上司は
Subject inside the time clause: 部下が

What does 病気で mean here, and why is the particle で used?

病気で here means “because of illness / due to being sick.”

The particle で can mark a reason or cause when attached to a noun:

病気で入院する – to be hospitalized due to illness
事故で亡くなる – to die in an accident / due to an accident

So 病気で入院している literally is:

is hospitalized because of illness.

Using something like 病気に入院する would be ungrammatical; 病気で is the natural pattern.

What nuance does 入院している have compared to just 入院する?

入院する simply means “to be hospitalized / to go into the hospital.”

入院している is te-form + いる, which:

grammatically: indicates a continuing state resulting from an action
nuance: “is in the state of being hospitalized” = “is (currently) in the hospital.”

So:

部下が入院する → the subordinate gets admitted (at some point)
部下が入院している → the subordinate is (currently) in the hospital

In this sentence, we need the idea of “while they are (in that state)”, so 入院している間 = while they are hospitalized.

What exactly does 間 mean here, and how is it different from 間に?

間 (あいだ) here means “the period (of time) while … / during the time when …”.

部下が病気で入院している間
→ the time during which the subordinate is in the hospital due to illness

間 vs 間に (in broad terms):

間: focuses on the entire duration of something
- テレビを見ている間、電話をしませんでした。
  While I was watching TV (for that whole time), I didn’t make calls.
間に: often highlights that something happens at some point within that span
- テレビを見ている間に、友達が来ました。
  While I was watching TV, at some point my friend came.

In your sentence, the boss’s request is about the whole period of hospitalization, so 間 (with は) is appropriate.

Why is there another は after 間 in 間は? Can a sentence have two は?

Yes, a sentence can have more than one は, but they function at different levels.

上司は – the main topic of the entire sentence
…間は – the time period is being used as a secondary / contrastive topic

間は here can be understood as:

部下が病気で入院している間は
→ As for the time while the subordinate is hospitalized…

This structure sets up a condition like “at least during that period … (don’t do X).” That’s why 間 gets a は, to highlight that specific time span.

What does 無理をする mean, and how do we get 無理をさせない from it?

無理をする literally: “to do the impossible / to overdo things / to push oneself too hard.”
Commonly used as:

あまり無理をしないでください。
Please don’t overdo it / don’t push yourself too hard.

The verb させる is the causative form of する:

する → させる: make/let someone do (something)

So:

無理をさせる → make someone overdo it / force someone to push themselves
無理をさせない → not let/make someone overdo it / avoid forcing someone to push themselves

In this sentence, 無理をさせない = not make (the subordinate) overdo things / not overwork them.

Why does the sentence say 無理をさせない instead of 無理をしない?

無理をしない: not to overdo it (oneself)
無理をさせない: not to make someone else overdo it

Here, the boss is asking everyone not to push the sick subordinate too hard. So the key idea is:

“Don’t make them work too hard”
rather than
“Don’t yourself work too hard.”

Hence the causative form させない, focusing on what you do to another person, is more accurate for this meaning.

Who is making whom do what in 無理をさせないように? The roles are a bit confusing.

Let’s unpack the roles:

Overall, 上司は皆に頼みました
→ The boss asked everyone.

What did the boss ask them to do (or not do)?

Implied structure:
（皆は）部下に無理をさせない
→ (Everyone) should not make the subordinate overdo it.

So:

“causer” (the one potentially making someone overdo it): 皆
“person who would be overworked”: 部下 (implied after 無理をさせない)

The sentence does not explicitly say 部下に無理をさせない, but in context it’s clear that the person who shouldn’t be overworked is the hospitalized subordinate.

What does ように do in 無理をさせないように皆に頼みました?

V-ないように頼む is a common pattern:

V-ないように頼む → ask someone not to V

Here:

無理をさせないように頼む
→ ask (them) so that they *don’t make (someone) overdo it
→ more naturally: ask them **not to make (someone) overdo it.*

So ように connects the desired state / action (無理をさせない) with the verb 頼みました, giving the sense of “requesting that X (not) happen.”

Why is it 皆に頼みました and not 皆を頼みました?

The verb 頼む behaves like “ask (a person) to do something”:

人に頼む – to ask a person / to request of a person

So:

皆に頼みました
→ asked everyone (to do something)

If you said 皆を頼みました, it would sound like:

人を頼む – to rely on / entrust someone with something
→ I entrusted everyone (with something) / I relied on everyone

That’s a different pattern and meaning. In this sentence we need the “ask someone (に)” pattern, so 皆に頼みました is correct.

Could you say 皆さんに頼みました instead of 皆に頼みました? Any difference?

Yes, you could say 皆さんに頼みました.

皆: plain, neutral “everyone” (can be casual to neutral depending on context)
皆さん: more polite / respectful, often used when addressing or referring politely to a group

皆に頼みました is already fine in many contexts, especially in written narrative.
皆さんに頼みました highlights politeness toward the group a bit more, fitting for formal speech or a boss speaking respectfully about “everyone.”

Is the word order fixed, or could we move parts around, like putting 皆に earlier?

Japanese word order is relatively flexible as long as you keep particle relationships clear.

For example, you could say:

上司は皆に、部下が病気で入院している間は無理をさせないように頼みました。
上司は、部下が病気で入院している間は、皆に無理をさせないように頼みました。

Both are grammatical.

However:

The core clause 無理をさせないように頼みました tends to stay together.
The time expression 部下が病気で入院している間は usually appears before the main action it modifies.

So you can shuffle 皆に and the time phrase somewhat, but you wouldn’t normally break 無理をさせないように away from 頼みました.

What is the politeness level of 頼みました, and how would it change in casual speech?

頼みました is the polite past form (～ます form):

頼む → 頼みます (polite non-past) → 頼みました (polite past)

Politeness level: the standard desu/masu politeness, suitable for narration, business, or talking to someone not very close.

In more casual speech (e.g. between close colleagues or friends), you might say:

上司は、皆に頼んだ。
(The) boss asked everyone.

Same meaning, but drops the polite ～ます style.