seki ga deru node, kyou ha hayame ni nemasu.

せきが出る is a very common Japanese way to say that someone has a cough or starts coughing.

Literally, it is closer to a cough comes out than to the English phrasing I cough.

• せき = cough
• 出る = to come out / to appear

So Japanese treats the cough as the thing that appears or comes out.

This is a natural set expression, so even if it feels strange from an English point of view, it is the normal way to say it in Japanese.

In せきが出る, the particle が marks せき as the thing that appears or occurs.

Think of it like this:

• せきが出る = a cough comes out / a cough occurs

With verbs like 出る, が often marks the thing that appears, happens, or comes out.

So this is not a direct object in the English sense. That is why を is not used here.

This is very common in Japanese.

When a verb comes before ので, it usually appears in its plain form:

• せきが出るので
• 雨が降ったので
• 時間がないので

The main sentence can still be polite:

• せきが出るので、今日は早めに寝ます。

So the sentence mixes:

• a plain-form verb inside the reason clause
• a polite main verb at the end

That is normal Japanese grammar, not an inconsistency.

ので gives a reason or cause, like because or so.

Here:

• せきが出るので = because I have a cough / because I’m coughing

Compared with から, ので usually sounds:

• a little softer
• more explanatory
• a bit more polite or less direct

So:

• せきが出るので、早めに寝ます。 = softer, explanatory
• せきが出るから、早めに寝ます。 = also correct, but slightly more direct or casual

In a sentence like this, ので sounds very natural.

は marks 今日 as the topic: as for today or today, ...

So:

• 今日は早めに寝ます。 = As for today, I’ll go to bed early.

You can think of 今日は as setting the frame for the statement.

Sometimes 今日早めに寝ます is possible in casual speech, but 今日は is very natural because it highlights today in contrast to other days.

早め means somewhat early or earlier than usual.

It comes from 早い meaning early, and -め adds the nuance of a bit or on the early side.

So:

• 早い = early
• 早め = relatively early / a little earlier than normal

In this sentence, 早めに寝ます does not necessarily mean very early. It usually means I’ll go to bed a bit earlier than usual.

In 早めに寝ます, the に turns 早め into an adverbial expression, telling us how or in what way the action is done.

So:

• 早めに寝ます = go to bed early / go to bed a bit earlier than usual

This use is common with words like:

• 早めに = early / earlier than usual
• 多めに = in a larger amount
• 少なめに = in a smaller amount

So the に here is what makes 早め modify the verb 寝ます.

In this sentence, 寝ます is best understood as go to bed.

That is because 早めに寝ます usually refers to the time you go to bed, not the act of being asleep.

Compare:

• 寝る = to go to bed / to lie down to sleep
• 眠る = to sleep

In everyday conversation, 寝る is very commonly used when talking about bedtime:

• もう寝ます。 = I’m going to bed now.
• 今日は早めに寝ます。 = I’ll go to bed early today.

So here, 寝ます is the natural choice.

Yes. Japanese very often omits the subject when it is clear from context.

So even though there is no explicit I, the sentence naturally means something like:

• Because I have a cough, I’ll go to bed early today.

Japanese often leaves out words like I, you, or he/she if the listener can easily understand who is meant.

That is why the sentence sounds complete without a stated subject.

The comma helps separate the reason clause from the main statement:

• せきが出るので、今日は早めに寝ます。

It is similar to how English often pauses after a phrase like because I have a cough.

The comma is helpful and very natural here, especially because the first part is a setup for the main action. In some short sentences, Japanese punctuation can be flexible, but this comma is standard and easy to read.