kyou ha karada ga darui node, gogo ha ie de yasumukoto ni simasu.

Japanese often leaves out the subject when it is obvious from context.

In 今日は体がだるいので、午後は家で休むことにします, the person speaking is clearly talking about their own condition and their own decision, so 私は is unnecessary.

A more explicit version would be:

私は、今日は体がだるいので、午後は家で休むことにします。

But in natural Japanese, that usually sounds more heavy or more contrastive than needed.

Here, は marks them as topics:

• 今日は = as for today / today
• 午後は = as for the afternoon / this afternoon

This helps organize the sentence:

• Today, my body feels sluggish, so...
• This afternoon, I’ll rest at home.

Also, time words like 今日, 明日, 昨日, and often 午後 do not always need に. In this sentence, the speaker is not just giving a time point—they are setting up topics, so は is natural.

Because these are the kinds of time expressions that often appear without に, especially when they are broad or relative times like:

• 今日 = today
• 明日 = tomorrow
• 昨日 = yesterday
• 今朝 = this morning

Also, in this sentence, は is being used to topicalize them.

You could say something like 午後に休みます in some contexts, but 午後は has a slightly different feel: it frames the afternoon as the part of the day being discussed.

が marks 体 as the subject of the adjective だるい.

So 体がだるい means:

• my body feels sluggish
• I feel physically run-down

This is the most neutral way to describe the condition.

If you said 体はだるい, it would sound more contrastive, as if you were saying something like:

• My body feels sluggish, but...
• As for my body, it feels sluggish

So が is the normal choice here.

だるい describes a feeling of physical sluggishness, heaviness, or lack of energy.

It is not exactly the same as:

• 疲れた = tired
• 眠い = sleepy
• 痛い = painful

だるい suggests something more like:

• I feel drained
• My body feels heavy
• I feel physically blah

So 体がだるい is a very natural Japanese way to say you feel physically unwell in a vague, low-energy way.

Both ので and から can mean because / since, but ので is usually softer and a bit more explanatory.

• だるいので = because I feel sluggish / since I feel sluggish
• だるいから = also correct, but a little more direct

In this sentence, ので sounds calm and natural, especially when explaining the reason for a decision.

A rough comparison:

• ので: softer, more objective, more explanatory
• から: more direct, more personal/assertive

Because で marks the place where an action happens.

• 家で休む = rest at home

Here, 休む is the action, and home is the location where that action takes place, so で is correct.

By contrast, に is often used for:

• destination: 家に帰る = go home
• existence: 家にいる = be at home

So:

• 家で休む = correct
• 家に休む = unnatural

Because this is the grammar pattern verb dictionary form + ことにする.

So:

• 休む = to rest
• 休むこと = the act of resting
• 休むことにする = to decide to rest

The こと turns the verb into something noun-like, which allows it to fit into this fixed pattern.

You cannot simply replace 休むことにします with 休みます if you want the same meaning, because 休みます only means I rest / I will rest, while 休むことにします adds the idea of making a decision.

～ことにします means decide to do ~.

In this sentence:

午後は家で休むことにします = I’ve decided to rest at home this afternoon = I think I’ll rest at home this afternoon

It shows the speaker is making a voluntary decision.

This is different from:

• ～ことになります = it has been decided that... / it turns out that...

Compare:

• 休むことにします = I decide to rest
• 休むことになります = it has been decided that I will rest / I will have to rest

So にします is the speaker’s own choice.

This is very normal in Japanese.

Inside a sentence, forms before things like ので and こと are often plain:

• だるいので
• 休むことにします

The politeness of the whole sentence is mainly shown at the end, with します.

So this is not mixing styles incorrectly. It is standard grammar.

You could think of it like this:

• non-final parts of the sentence often use plain forms
• the final verb shows the overall politeness level

Yes, it can.

は often has a mild contrastive nuance, so 午後は can feel like:

• as for the afternoon
• at least in the afternoon
• the afternoon, specifically

In context, it may suggest something like:

• maybe the speaker was active earlier, but not in the afternoon
• maybe only the afternoon plan is changing

However, the contrast does not have to be strong. Sometimes は is just the natural way to present the topic.

Not really. It feels incomplete, because ので usually introduces a reason that leads to something else.

So:

• 今日は体がだるいので... = Because I feel sluggish today...

The listener expects the result or conclusion after that.

That conclusion is:

午後は家で休むことにします。

So the full structure is:

• reason: 今日は体がだるいので
• decision/result: 午後は家で休むことにします