mausu ga ugokanai node, atarasii mausu ni kaemasu.

In マウスが動かない, が marks マウス as the thing that is not working / not moving.

A simple way to think about it:

• が points to the thing affected by the verb or adjective
• は sets up a topic, often with contrast

So:

• マウスが動かない = The mouse doesn’t work / move
• マウスは動かない can sound more like As for the mouse, it doesn’t work or the mouse (unlike something else) doesn’t work

In many everyday sentences like this, が sounds more natural when you are simply stating the problem.

動かない is the negative plain form of 動く.

• dictionary form: 動く = to move
• negative plain form: 動かない = does not move / won’t move

This is how -u verbs are negated:

• 書く → 書かない
• 行く → 行かない
• 動く → 動かない

So grammatically, 動かない is just 動く changed into the plain negative.

Yes, literally 動かない means doesn’t move.

But in context, with a computer mouse, Japanese often uses 動く / 動かない to mean:

• works / functions
• doesn’t work / isn’t responding

So for a mouse:

• マウスが動かない can mean the cursor doesn’t move
• or more naturally in English, the mouse isn’t working

This is a very common kind of context-based meaning in Japanese.

ので means because, since, or so.

In this sentence:

• マウスが動かないので = because the mouse doesn’t work

Then the second part gives the result:

• 新しいマウスに替えます = I’ll switch to a new mouse

So the whole sentence is structured as:

• Reason + ので + result

It often sounds a little softer or more matter-of-fact than から.

Both can mean because, but they feel a little different.

• から is more direct and straightforward
• ので is often softer, more explanatory, and slightly more formal/polite in tone

Compare:

• マウスが動かないから、新しいマウスに替えます。
• マウスが動かないので、新しいマウスに替えます。

Both are correct.
The ので version sounds a bit more like giving a reason calmly or politely.

Here, に marks the thing being changed to.

• 新しいマウスに替えます = change/switch to a new mouse

This is a common pattern with 替える:

• AをBに替える = change/replace A to/with B

In this sentence, the old mouse is not said explicitly, but it is understood.

So the full idea is something like:

• (古いマウスを) 新しいマウスに替えます
• I’ll replace the old mouse with a new mouse

Yes, logically there is an omitted object.

With 替える, you often get a pattern like:

• 古いマウスを新しいマウスに替えます
• replace the old mouse with a new mouse

In your sentence, the old mouse is obvious from context, so Japanese leaves it out:

• マウスが動かないので、新しいマウスに替えます。

This kind of omission is very common in Japanese. If the listener can easily guess the missing information, it is often not said.

替えます is the polite -ます form of 替える.

Here it means:

• to replace
• to switch
• to change one thing for another

It is often used when one item is being swapped for another equivalent item.

Examples:

• 電池を新しいのに替える = replace the battery with a new one
• 古いノートを新しいノートに替える = replace the old notebook with a new one

So in this sentence, 替えます means I’ll replace it / switch to a new one.

These words are related, but not identical.

• 替える: replace one thing with another, often of the same type
  
  • マウスを新しいマウスに替える
• 変える: change something in a broader sense
  
  • change plans, change color, change method, etc.
• 交換する: exchange/replace, often more formal or mechanical
  
  • 部品を交換する = replace a part

So in this sentence, 替える fits well because you are swapping one mouse for another mouse.

In Japanese, adjectives come before the noun they describe.

So:

• 新しいマウス = new mouse
• 古いパソコン = old computer
• 大きい机 = big desk

This works a lot like English word order for basic adjectives.

Also, 新しい is an i-adjective, so it stays directly before the noun without needing anything in between.

Japanese often repeats nouns where English would use pronouns like it.

English:

• The mouse doesn’t work, so I’ll replace it with a new one.

Japanese often prefers:

• マウスが動かないので、新しいマウスに替えます。

This is natural in Japanese. Pronouns are used less often than in English, and repeating the noun can sound clearer and more normal.

The sentence follows a very common Japanese pattern:

• Reason + result
• [マウスが動かないので]、[新しいマウスに替えます]

More literally:

• mouse + subject marker + doesn’t work + because, new mouse + to + replace

Japanese verbs usually come at the end of their clause, so:

• 動かない ends the reason clause
• 替えます ends the main clause

That is why the sentence feels “backwards” compared with English at first.

Because 替えます is in the polite non-past form, it can mean:

• I will replace it
• I am replacing it
• sometimes a general future/intention depending on context

In this sentence, the most natural interpretation is:

• I’ll replace it with a new mouse

Japanese often leaves the subject unstated, so I is understood from context.

The comma separates the reason from the main action:

• マウスが動かないので、
• 新しいマウスに替えます。

It helps readability and makes the pause clearer.

Japanese commas are often more flexible than English commas, so sometimes you will see them and sometimes not in similar sentences. But here it is very natural to include one after the ので clause.