kossetusita ude wo ugokasu no de ha naku, hukusi de koteisuru.

A natural reading is:

こっせつした うで を うごかす のではなく、ふくし で こていする。

Word readings:

• 骨折した → こっせつした
• 腕 → うで
• 動かす → うごかす
• 副子 → ふくし
• 固定する → こていする

Because 骨折した modifies 腕.

In Japanese, modifiers come before the noun they describe. So:

• 骨折した腕 = the arm that was fractured / the broken arm

This is very normal Japanese word order. It works like a relative clause in English, but Japanese does not need words like that or which here.

So:

• 骨折した腕 literally = fractured arm

を marks the direct object of the verb.

Here, the verb is 動かす meaning to move something. The thing being moved is 腕.

So:

• 腕を動かす = to move the arm

That is why 腕 takes を.

They are related, but not the same.

• 動く = to move / to move on its own
• 動かす = to move something

So:

• 腕が動く = the arm moves
• 腕を動かす = to move the arm

In this sentence, someone is moving the arm, so Japanese uses the transitive verb 動かす.

Here, の turns the verb phrase into something noun-like.

• 腕を動かす = to move the arm
• 腕を動かすの = moving the arm / the act of moving the arm

This is needed because the sentence uses the pattern:

• A のではなく、B
• not A, but rather B

So the の helps package 腕を動かす into a unit that can be contrasted with 固定する.

のではなく means something like:

• not ... but ...
• rather than ...
• instead of ...

So in this sentence:

• 腕を動かすのではなく、副子で固定する
• Rather than moving the arm, immobilize it with a splint

It contrasts two actions:

1. moving the broken arm
2. fixing it with a splint

The second action is presented as the correct one.

Because ではなく is part of a standard contrastive pattern.

Breakdown:

• で connects to the nominalized phrase
• は adds contrast
• なく is the negative connective form of ない

Together, のではなく strongly sets up a contrast:

• not doing A, but doing B

The は is especially important for that contrastive feeling.

Here, で marks the means or tool used to do something.

So:

• 副子で固定する = to immobilize/fix it using a splint

This で is like:

• はしで食べる = eat with chopsticks
• ペンで書く = write with a pen

So 副子で means with a splint.

副子 means splint.

It is a medical term, so it is not an everyday beginner word. In ordinary conversation, many people may be more familiar with explanations like:

• 腕を板などで固定する
• to immobilize the arm with a board-like support

But in medical or first-aid language, 副子 is the proper term for splint.

固定する means to fix, to secure, or to immobilize.

In a medical context, it usually means:

• to keep something from moving
• to stabilize it

So in this sentence, 固定する does not just mean attaching something loosely. It means properly securing the broken arm so it does not move.

Japanese often uses the dictionary form in instructions, manuals, and medical explanations.

So 固定する here can sound like:

• a procedure step
• an instruction
• a recommendation
• a textbook-style statement

It is less directly forceful than a strong imperative, but it still functions as an instruction in context.

Compare:

• 固定する = instructional/manual style
• 固定してください = polite request
• 固定しなさい = direct command
• 固定せよ = very formal/strong command

The subject is omitted, which is very common in Japanese.

Depending on context, it could mean:

• you should immobilize it
• one should immobilize it
• medical staff should immobilize it

Japanese often leaves out subjects when they are obvious from context. In an instructional or first-aid sentence like this, the understood meaning is something like:

• Do not move the broken arm; instead, immobilize it with a splint.

It mainly sets up a contrast:

• not moving the broken arm
• but immobilizing it with a splint

So the emphasis is: the correct response is immobilization, not moving it around.

In practical English, this often comes across as:

• Do not move the broken arm; instead, splint it.

So yes, it strongly implies that moving it is the wrong thing to do.

Yes, that is possible, but the nuance is a little different.

• 腕を動かさないで、副子で固定する

  • without moving the arm, immobilize it with a splint
  • or don’t move the arm; immobilize it with a splint

• 腕を動かすのではなく、副子で固定する

  • rather than moving the arm, immobilize it with a splint
  • stronger contrast between the wrong action and the right action

So the original sentence is especially good when the speaker wants to contrast two approaches.

No, normal Japanese is usually written without spaces like this:

骨折した腕を動かすのではなく、副子で固定する。

The spaces are probably included only to make it easier for learners to see the parts of the sentence.

That is common in textbooks or teaching materials, but not in ordinary Japanese writing.