senaka ga itai node, kyou ha ie de yasumimasu.

が marks 背中 as the thing experiencing the state 痛い.

In Japanese, body parts and feelings are often marked this way:

• 頭が痛い = My head hurts.
• お腹が痛い = My stomach hurts.
• 背中が痛い = My back hurts.

A native English speaker may want to think I am the subject, but Japanese often focuses on the body part instead.

You could say 背中は痛い, but that would give extra contrast, like As for my back, it hurts.

痛い is an い-adjective, not a noun or regular verb.

So:

• 背中が痛い = My back hurts.

You do not say:

• ✕ 背中が痛です

because 痛 by itself is not used that way.

There is a verb 痛む meaning to ache / to hurt, so 背中が痛みます is possible, but 背中が痛い is the more common, basic pattern for saying a body part hurts.

Before ので, Japanese often uses the plain form:

• 痛いので = because it hurts

Even in polite speech, the part before ので is very often plain, while the sentence ends politely:

• 背中が痛いので、今日は家で休みます。

This is completely natural.

You may also hear:

• 背中が痛いんです。
• 背中が痛いので、休みます。

Using ですので is possible in some contexts, but here 痛いので sounds more standard and natural.

ので means because, since, or so. It gives the reason for the second part of the sentence.

So the structure is:

• 背中が痛いので = because my back hurts
• 今日は家で休みます = I’ll rest at home today

Compared with から, ので often sounds a little softer and more explanatory.

For example:

• 背中が痛いから、休みます。
• 背中が痛いので、休みます。

Both mean roughly the same thing, but ので can sound a bit more polite or less blunt.

は marks 今日 as the topic: as for today.

So:

• 今日は = today / as for today

This gives the sense that the speaker is talking specifically about today’s plan.

Compare:

• 今日は家で休みます。 = As for today, I’ll rest at home.
• 今日家で休みます。 = I’ll rest at home today.

Both are possible, but 今日は sounds a bit more natural when setting the scene or topic.

で marks the place where an action happens.

Here, the action is 休みます:

• 家で休みます = rest at home

So:

• で = where the action takes place

Compare:

• 家にいます = be at home
• 家で休みます = rest at home

A common learner question is why it is not に. The reason is that 休みます is treated as an action happening in that place, so で is the natural choice.

Here, 休みます means to rest or to take it easy.

So:

• 家で休みます = I’ll rest at home.

Depending on context, 休みます can also mean:

• to take a day off
• to be absent
• to go to bed / sleep for a while

But in this sentence, because of 家で, it most naturally means rest at home.

Japanese often leaves out subjects when they are obvious from context.

So instead of saying:

• 私は背中が痛いので、今日は家で休みます。

Japanese usually just says:

• 背中が痛いので、今日は家で休みます。

The listener naturally understands that the speaker is talking about themself.

This omission is very common and very natural in Japanese.

背中 is read せなか.

It usually means:

• back
• especially the back of the body, from shoulders down toward the waist

So 背中が痛い means my back hurts.

It is different from words like:

• 腰 = lower back / hips / waist area
• 肩 = shoulder

So if the pain is specifically in the lower back, someone might also say:

• 腰が痛い = My lower back hurts.

Yes. This is very common in Japanese.

In this sentence:

• 痛いので = plain form in the reason clause
• 休みます = polite form at the end

The politeness level of the whole sentence is mainly shown by the final verb. Because the sentence ends with 休みます, the overall sentence is polite.

This kind of mix is normal and natural.

Yes, in many situations you could.

• 家で休みます
• うちで休みます

Both can mean I’ll rest at home.

But there is a nuance:

• 家 is the standard, neutral word for house / home
• うち is often more casual and conversational

So in a polite sentence like this, 家で sounds especially standard and safe.

The comma helps separate the reason part from the main action part:

• 背中が痛いので、 = because my back hurts,
• 今日は家で休みます。 = I’ll rest at home today.

It makes the sentence easier to read, especially in writing.

Japanese commas are somewhat flexible, so you may also see sentences without one, but here the comma is very natural.