kanozyo ha maiasa manindensya de tuukinsuru node, kaisya ni tuku mae ni sukosi tukarete simau.

A natural reading is:

彼女 → かのじょ
毎朝 → まいあさ
満員電車 → まんいんでんしゃ
通勤する → つうきんする
ので → ので
会社 → かいしゃ
着く → つく
前に → まえに
少し → すこし
疲れてしまう → つかれてしまう

Also, 彼女 can sometimes mean girlfriend, but in this sentence it means she / her.

は marks 彼女 as the topic of the sentence: we are talking about her.

So 彼女は feels like:

• As for her...
• She...

If you used が, it would put more focus on who is doing the action, as if identifying the subject:

• 彼女が = she is the one who...

In a normal descriptive sentence like this, は is the most natural choice.

Many common time expressions in Japanese do not need a particle. 毎朝 is one of them.

So:

• 毎朝 満員電車で通勤する = commute by crowded train every morning

This is very normal. The same thing often happens with words like:

• 今日
• 明日
• 毎日
• 去年

You can sometimes see time words with particles in other contexts, but here no particle is the standard form.

満員電車 is a very common compound noun meaning a packed train or a crowded commuter train.

So it works as one unit:

• 満員 = full / packed
• 電車 = train
• 満員電車 = packed train

満員の電車 is not impossible, but 満員電車 is the much more natural set phrase.

Here, で marks the means or method of commuting.

So:

• 満員電車で通勤する = commute by packed train

This is the same pattern as:

• 電車で行く = go by train
• バスで通学する = commute to school by bus

So で is not marking location here; it is marking how she commutes.

通勤する specifically means to commute to work.

It is more specific than 行く, which just means to go.

Compare:

• 会社に行く = go to the company / go to work
• 会社に通勤する = commute to work regularly

Because this sentence is about her daily routine, 通勤する is a very good fit.

Both can mean because, but the nuance is a little different.

• ので sounds a bit more explanatory, neutral, and softer
• から can sound a bit more direct or assertive

So:

• 満員電車で通勤するので、疲れてしまう feels like a calm explanation
• 満員電車で通勤するから、疲れてしまう is also possible, but slightly more direct

In this sentence, ので matches the explanatory tone well.

Japanese often puts the reason clause first and the main result later.

So the structure is:

彼女は
毎朝 満員電車で 通勤するので
会社に着く前に 少し 疲れてしまう

Very literally, this is something like:

• As for her, because she commutes every morning on a packed train, before arriving at the company, she ends up getting a little tired.

Also, notice that 彼女 is not repeated in the second half. Japanese often omits the subject when it is already understood. The subject of 疲れてしまう is still her.

They are doing two different jobs.

1. 会社に着く
   Here, に marks the destination of 着く:

   • 会社に着く = arrive at the company

2. 前に
   Here, に marks the time point before something:

   • 着く前に = before arriving

So the two に are unrelated in function, even though they look the same.

With 前に, Japanese normally uses the dictionary form of the verb.

So:

• 着く前に = before arriving
• 食べる前に = before eating
• 寝る前に = before sleeping

This is because the action has not happened yet at that point.

So 会社に着く前に means before she arrives at the company.

少し means a little or slightly, and here it modifies the idea of becoming tired.

So:

• 少し疲れてしまう = she ends up getting a little tired

It does not mean she is completely exhausted. It suggests a mild degree of tiredness.

しまう adds nuance. In this sentence, it suggests something like:

• end up getting tired
• become tired unfortunately
• get tired before she means to

So:

• 疲れる = get tired
• 疲れてしまう = end up getting tired / unfortunately get tired

Here, しまう gives the sentence a slightly negative or regrettable feeling, which fits the idea that the crowded commute wears her out.

Not necessarily in a super literal, exact sense. It more naturally means:

• by the time she gets to work, she is already a little tired
• she gets tired even before arriving at work

So the focus is on the idea that the commute is tiring so early that she is already worn out before the workday really starts.