sono hako ha omokatta node, ude dake de naku yubi mo tukaremasita.

Because the sentence is talking about a past situation.

• 重い = is heavy
• 重かった = was heavy

So その箱は重かったので means because that box was heavy.
The reason clause is describing what was true at that time, so the adjective is put in the past form.

ので means because or so.

In this sentence:

• その箱は重かったので = because that box was heavy
• 腕だけでなく指も疲れました = not only my arms but my fingers got tired too

Compared with から, ので often sounds a little softer, more explanatory, and less direct. It is very common when giving a reason in a neutral or polite way.

The は marks 箱 as the topic.

So その箱は means something like:

• as for that box
• that box

It sets up what the sentence is talking about, and then the rest of the sentence says something about it: it was heavy, and as a result, the speaker’s arms and fingers got tired.

だけでなく means not only ... but also ...

Pattern:

• A だけでなく B も ...
• not only A, but B also ...

In this sentence:

• 腕だけでなく
• not only (my) arms

Then the sentence continues with:

• 指も
• fingers too

So together:

• 腕だけでなく指も疲れました
• not only my arms but my fingers got tired too

Because も means also or too.

In the pattern A だけでなく B も, the も marks the second item as something added:

• not only A
• but B too

So:

• 腕だけでなく指も疲れました
• Not only my arms, but my fingers got tired too

Without も, the contrast/addition would feel incomplete.

Japanese often leaves out things that are obvious from context, especially with body parts.

So instead of saying:

• 私の腕
• 私の指

it is very natural to simply say:

• 腕
• 指

Since the sentence is about becoming tired from carrying the box, it is obvious that these are the speaker’s arms and fingers.

疲れました is the polite past form of 疲れる:

• 疲れる = to get tired
• 疲れた = got tired / was tired (plain past)
• 疲れました = got tired / was tired (polite past)

This sentence is written in polite style, which is why it ends with ました.

疲れています would mean is tired / are tired in a more current state-focused way, which is not the best fit here. The sentence is describing what happened as a result of the box being heavy, so 疲れました works naturally.

Because 腕 means arm, while 手 usually means hand.

Here the sentence contrasts:

• 腕 = arms
• 指 = fingers

The idea is that lifting or carrying the heavy box tired not just the larger part of the body doing the work (arms), but even the smaller parts gripping it (fingers).

If 手 were used, the meaning would shift more toward hands rather than arms.

Yes. They are very similar, but the tone is different.

• だけでなく = more formal / neutral / written-sounding
• だけじゃなく = more casual / conversational

So this sentence could casually become:

• その箱は重かったので、腕だけじゃなく指も疲れました。

The meaning stays the same.

その means that and refers to something near the listener, or something already mentioned in the conversation.

Japanese demonstratives work like this:

• この = this (near the speaker)
• その = that (near the listener / previously mentioned)
• あの = that over there (away from both)

So その箱 means that box. It suggests the box is already known in the conversation or context.

No. Normal Japanese writing does not usually use spaces between words.

So the standard form is:

その箱は重かったので、腕だけでなく指も疲れました。

Spaces are often added in learning materials to make the structure easier to see.

It breaks down like this:

• その箱は = as for that box
• 重かったので = because it was heavy
• 腕だけでなく = not only my arms
• 指も = but my fingers too
• 疲れました = got tired

So the overall structure is:

[Topic] + [Reason] + [not only A but also B] + [result]

That makes the sentence easy to understand as:

That box was heavy, so not only my arms but my fingers got tired too.