hanamizu dake de naku seki mo deru node, kyou ha gaisyutusimasen.

だけでなく means not only ... but also ....

In this sentence:

• 鼻水だけでなく = not only a runny nose
• せきも出る = a cough also appears / I’m also coughing

So the pattern is:

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

A very common translation pattern is:

• X だけでなく Y も = not only X, but also Y

The も matches the earlier だけでなく.

The structure is:

• 鼻水だけでなく、せきも出る
• not only runny nose, but cough also occurs

When Japanese says A だけでなく B も, the も helps mark the also part.

So:

• だけでなく = not only
• も = also / too

They work together.

Because in Japanese, 出る is very commonly used for symptoms that come out, appear, or occur.

So:

• 鼻水が出る = to have a runny nose
• せきが出る = to cough / to have a cough

This may sound a little unusual if you try to translate word-for-word into English, because English usually says:

• I have a runny nose
• I have a cough
• I’m coughing

But Japanese often describes symptoms as something that comes out or appears.

This is very natural Japanese.

せきをする and せきが出る can both relate to coughing, but they feel a little different.

• せきをする = to cough
• せきが出る = a cough comes out / I have a cough / I start coughing

せきが出る is especially common when talking about a symptom or physical condition. It often sounds more like the symptom is occurring.

So in a sentence about illness, せきが出る is very natural.

Both ので and から can mean because, but they have slightly different nuance.

• ので often sounds a bit more explanatory, softer, or more polite
• から can sound a bit more direct or assertive

Here:

• 鼻水だけでなくせきも出るので、今日は外出しません。
• Because I have not only a runny nose but also a cough, I won’t go out today.

Using ので fits well because the speaker is giving a calm reason for their decision.

The は marks today as the topic or contrastive time frame.

So:

• 今日は外出しません = As for today, I’m not going out
• or more naturally, I’m not going out today

The は can also give a slight sense of contrast, like:

• today, at least, I won’t go out

It does not always need strong contrast, but it often helps set the scene: as for today...

This is very normal Japanese sentence structure.

In Japanese, the main sentence ending usually determines the overall politeness level. Here, the sentence ends with:

• 外出しません = polite negative

But verbs inside dependent parts of the sentence, such as before ので, often stay in plain form:

• せきも出るので = because I also have a cough

So it is completely natural to mix:

• plain form inside the reason clause
• polite form at the sentence end

This is standard Japanese.

This is a great question, because the underlying idea is really about symptoms occurring.

You can think of the sentence as related to:

• 鼻水が出る
• せきも出る

But in the pattern A だけでなく B も 出る, the first item often appears without が before だけでなく:

• 鼻水だけでなく、せきも出る

This is a very natural compressed structure meaning:

• not only runny nose, but cough also occurs

If you expanded it, you might get something like:

• 鼻水が出るだけでなく、せきも出る
• Not only do I have a runny nose, but I also have a cough

That version is also grammatical, but the original sentence is shorter and very natural.

Yes, the subject is omitted, which is extremely common in Japanese.

From context, the subject is understood to be something like:

• I
• or possibly the speaker

So the full idea is:

• (I) have not only a runny nose but also a cough, so (I) won’t go out today.

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

外出する means to go out, especially in the sense of going out of the house or going out somewhere.

So:

• 外出しません = will not go out / am not going out

It is a slightly more formal or written-sounding word than very casual alternatives, but it is common and useful.

Yes, that is a good natural translation.

A few possible English renderings are:

• I not only have a runny nose, but I’m also coughing, so I’m not going out today.
• Since I have a runny nose and a cough, I’m not going out today.
• Because I have not only a runny nose but also a cough, I won’t go out today.

The last one is closer to the Japanese structure, but the others may sound more natural in everyday English.

Yes. だけでなく can connect more than just nouns.

For example:

• 日本語だけでなく、韓国語も勉強しています。
  • I’m studying not only Japanese but also Korean.

It can also attach to larger phrases:

• 行くだけでなく、手伝いもします。
  • I’ll not only go, but I’ll help too.

In your sentence, it is used with a noun:

• 鼻水だけでなく

That is one of its most common uses.