kyou ha karada ga darui node, muri ni undousurukoto ha nai.

They are marking different things.

• 今日 は = as for today / today

  • は marks the topic.
  • It sets the time frame: speaking of today...

• 体 が だるい = the body feels sluggish / my body feels heavy

  • が marks the thing that has the condition described by だるい.

So the structure is roughly:

• 今日は = today,
• 体がだるい = my body feels sluggish,
• ので = so / because

A natural way to understand it is: As for today, my body feels sluggish, so...

体がだるい is the more neutral way to state the condition itself: the body feels sluggish.

With adjectives like だるい, が often marks what has that quality.

• 体がだるい = my body feels sluggish
• 頭が痛い = my head hurts
• 喉が渇いた = I’m thirsty

If you said 体はだるい, it would sound more contrastive, like:

• As for my body, it feels sluggish
  (possibly implying something else does not, or contrasting it with another thing)

So が is the natural choice here.

だるい means something like:

• sluggish
• listless
• heavy
• worn out
• aching and low-energy

It is often used when your body feels tired in a physical, uncomfortable way—not just ordinary sleepiness.

So 体がだるい is not simply I’m sleepy. It is more like:

• My body feels heavy
• I feel physically sluggish
• I feel run-down

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

• ので sounds a bit more soft, explanatory, and less direct
• から can sound more direct or assertive

So:

• 体がだるいので、無理に運動することはない。
  • Since my body feels sluggish, there’s no need to force exercise.

If you used から instead:

• 体がだるいから、無理に運動することはない。

That is also grammatical, but ので fits well when calmly giving a reason.

無理 is a noun, and に turns it into an adverb-like expression.

• 無理 = impossibility / unreasonable strain / forcing
• 無理に = forcibly / against one’s limits / by pushing oneself too hard

So:

• 無理に運動する = to exercise by forcing oneself
• 無理に食べる = to force oneself to eat
• 無理に行く = to go even though it’s too much / unreasonable

Here it means something like: to exercise even though you’re not in good condition

This is a very common Japanese pattern:

• dictionary form + ことはない

It means:

• there is no need to...
• you don’t have to...

So:

• 運動することはない = there is no need to exercise
• more specifically here: there’s no need to force yourself to exercise

This pattern does not simply mean there is no such thing as exercising. It is an idiomatic grammar pattern expressing lack of necessity.

More examples:

• 急ぐことはない。 = There’s no need to hurry.
• 心配することはない。 = There’s no need to worry.
• 謝ることはない。 = You don’t need to apologize.

Because こと nominalizes the verb phrase—that is, it turns 運動する (to exercise) into something like exercising as a concept.

• 運動する = to exercise
• 運動すること = exercising / the act of exercising

The grammar pattern V-ることはない requires that nominalized form.

So the sentence uses:

• 運動することはない
• literally: as for exercising, there is no need

In natural English: There’s no need to exercise or You don’t have to exercise.

Not exactly.

• 運動することはない = there is no need to exercise
• It does not strongly forbid the action.

This is softer than:

• 運動してはいけない = you must not exercise
• 運動しないほうがいい = it would be better not to exercise

So the nuance here is: Since you feel physically sluggish today, you don’t need to force yourself to exercise.

It leaves room for choice, rather than giving a strict prohibition.

Japanese often leaves the subject unstated when it is clear from context.

In this sentence, the most natural subject is probably I:

• Today my body feels sluggish, so there’s no need to force myself to exercise.

But depending on context, it could also be advice to you:

• Since your body feels sluggish today, there’s no need to force yourself to exercise.

Japanese often relies on context rather than explicit pronouns like I or you.

The comma helps separate the reason part from the main statement.

• 今日は体がだるいので、 = Since my body feels sluggish today,
• 無理に運動することはない。 = there’s no need to force exercise.

Japanese commas are often used more flexibly than English commas, but here it makes the sentence easier to read.

Because it is followed by は, it is not just a simple time word. It is setting up today as the topic or frame.

• 今日 = today
• 今日は = as for today / today, at least

This can carry a slight nuance of limitation or contrast:

• Today I feel sluggish.
• Maybe other days are different.

So 今日は is slightly more than just today; it frames the whole statement from the perspective of today.

Yes. If the time is already clear from context, you could say:

• 体がだるいので、無理に運動することはない。

That would mean: Since my body feels sluggish, there’s no need to force exercise.

Adding 今日は makes the sentence more specific and situational: Today, since my body feels sluggish...

Literally, 体 means body, but in sentences like this it often refers to your physical condition.

So 体がだるい is not talking about the body in an abstract anatomical way. It means something more natural like:

• I feel physically sluggish
• My body feels heavy
• I’m feeling run-down

In English, we often translate it less literally for naturalness.

Yes, and the meaning would be very similar:

• 運動することはない。
• 運動しなくていい。

Both can mean: You don’t need to exercise.

However, there is a nuance difference:

• V-ることはない sounds a bit more formal, written, or advisory
• V-なくていい is more casual and conversational

So in a casual spoken situation, many people might say:

• 今日は体がだるいので、無理に運動しなくていい。

But the original sentence is perfectly natural, especially in written or explanatory style.