seki ga tomaranai node, asita no asa ni naika he iku tumori desu.

Literally, it means the cough does not stop.

• せき = cough
• 止まらない = does not stop

In natural English, this often becomes my cough won’t stop or I can’t stop coughing.

が marks せき as the thing that is doing the verb 止まる.

With 止まる, the thing that stops is marked with が:

• 雨が止まる = the rain stops
• せきが止まる = the cough stops

So せきが止まらない is literally the cough does not stop.

This is a very common question because 止まる and 止める are a pair:

• 止まる = to stop, to come to a stop (intransitive)
• 止める = to stop something (transitive)

Here, the cough is stopping on its own, so Japanese uses the intransitive verb:

• せきが止まらない = the cough won’t stop

If you used 止める, it would sound more like I do not stop the cough, which is not the intended idea.

ので means because or since.

In this sentence:

• せきが止まらないので = because my cough won’t stop

Compared with から, ので often sounds:

• a little softer
• a little more explanatory
• a little more formal or neutral

So this sentence has a calm, natural tone: Because my cough won’t stop, I plan to go...

明日 means tomorrow, but 明日の朝 specifically means tomorrow morning.

Breakdown:

• 明日 = tomorrow
• 朝 = morning
• 明日の朝 = tomorrow’s morning / tomorrow morning

The の links the two nouns, like tomorrow’s morning.

Not always. Time expressions in Japanese sometimes take に, and sometimes they do not.

So both of these are possible:

• 明日の朝に内科へ行くつもりです
• 明日の朝、内科へ行くつもりです

In many everyday sentences, に can be omitted with expressions like 明日, 今日, 来週, and often also with 明日の朝.

Using に here is not wrong. It just makes the time point a little more explicitly marked.

内科 means internal medicine or the internal medicine department.

In everyday English, depending on the situation, this might be translated as:

• the internal medicine department
• a physician
• a general medical clinic
• sometimes simply the doctor

In Japanese, going to 内科 suggests seeing a doctor for non-surgical internal health issues, such as fever, cough, stomach problems, and so on.

Yes, に could also be used here.

With verbs of movement like 行く, both へ and に can mark the destination:

• 内科へ行く
• 内科に行く

Difference:

• へ emphasizes the direction toward a place
• に emphasizes the destination itself

In many sentences, the difference is small, and both are natural.

つもりです means intend to do or plan to do.

So:

• 行くつもりです = I intend to go / I’m planning to go

It shows a personal intention or plan. It is stronger than just saying maybe I’ll go, but it does not necessarily mean the action is 100% fixed.

Because つもり attaches to the plain form of a verb, not the polite -ます form.

So:

• 行くつもりです = correct
• 行きますつもりです = incorrect

This is because つもり behaves like a noun, and the plain verb form modifies it:

• 行く
  • つもり = the intention to go

This pattern is very common:

• 日本へ行くつもりです = I intend to go to Japan
• 勉強するつもりです = I intend to study

This is completely normal in Japanese.

Japanese often uses:

• plain form inside a clause or before certain grammar patterns
• polite form at the end of the whole sentence

Here:

• 止まらないので: plain form before ので
• 行くつもりです: plain form before つもり, then polite です at the end

So the sentence is still polite overall because it ends politely.

Yes. せき can be written as 咳.

So the sentence could also be written:

• 咳が止まらないので、明日の朝に内科へ行くつもりです。

However, medical or somewhat uncommon kanji are often written in hiragana, so せき is very natural too.

Yes, it is natural and understandable.

A slightly more everyday version might be:

• せきが止まらないので、明日の朝、内科に行くつもりです。

That version sounds a little smoother to many speakers because:

• the に after 朝 is omitted
• に is used instead of へ, which is also very common

But the original sentence is still perfectly fine Japanese.