kono sinkei ga itai.

You’re right: standard Japanese normally writes this as この神経が痛い。 with no spaces.

The spaces are usually added in beginner materials to make the parts easier to see:

• この = this
• 神経 = nerve
• が = subject-marking particle
• 痛い = painful / hurts

So the spaces are a teaching aid, not normal orthography.

この is used when you put this directly before a noun.

• この神経 = this nerve

By contrast, これ stands alone:

• これ = this

So you can say:

• この神経が痛い。 = This nerve hurts.
• これは痛い。 = This hurts.

But これ神経 is not correct.

Because この already directly modifies the noun that follows it.

• この神経 = this nerve

You do not say このの神経.

A useful comparison:

• この + noun → この神経
• これ + の + noun is generally not how you say this noun in normal Japanese

So この works a bit like a built-in determiner that attaches straight to the noun.

Here, 神経 most naturally means a physical nerve in the body.

However, 神経 can also be used in other ways depending on context, such as:

• nerves in the sense of mental tension
• sensitivity
• expressions involving temperament or irritation

For this sentence, though, the most likely reading is the anatomical one: a specific nerve hurts.

In Japanese, body parts and things experiencing a condition are often marked with が.

So:

• 神経が痛い literally means something like the nerve is painful

This is a very common pattern with physical sensations:

• 頭が痛い = My head hurts.
• 歯が痛い = My tooth hurts.
• お腹が痛い = My stomach hurts.

English often makes the person the subject, but Japanese often makes the body part the thing marked with が.

Yes, but the nuance changes.

• この神経が痛い。 = neutral, identifying the thing that hurts
• この神経は痛い。 = as for this nerve, it hurts / this nerve is the painful one

Using は can sound more contrastive or topical, as if you are comparing it with something else:

• この神経は痛いけど、あの神経は大丈夫。
  This nerve hurts, but that one is fine.

So が is the more straightforward choice for a simple statement of pain.

Japanese often leaves out possession when it is obvious from context, especially with body parts.

So instead of saying:

• 私の神経が痛い

Japanese often just says:

• 神経が痛い

If the speaker is talking about their own body, listeners usually understand that automatically. In a medical setting, it could also refer to the patient’s nerve, depending on context.

This omission is very natural in Japanese.

No. 痛い is an i-adjective.

That can feel unusual to English speakers, because English says it hurts, using a verb. Japanese often expresses this idea with an adjective meaning painful.

So structurally, 神経が痛い is closer to:

• the nerve is painful

rather than:

• the nerve hurts

Even though the natural English translation is usually hurts.

Yes. 痛む is a verb meaning to ache / to hurt.

So both are possible:

• 神経が痛い
• 神経が痛む

Very roughly:

• 痛い focuses on the state or sensation of pain
• 痛む can sound a little more like the pain is occurring or aching

In many situations, both are understandable, but Xが痛い is an extremely common basic pattern learners should know well.

It is grammatically natural, but it sounds fairly specific.

People often say more general things in casual conversation, such as:

• ここが痛い。 = It hurts here.
• 歯が痛い。 = My tooth hurts.
• 腰が痛い。 = My lower back hurts.

Using 神経 sounds more medical, technical, or precise, such as when talking to a doctor or dentist, or when pointing to a specific nerve problem.

So the sentence is fine, but the word 神経 makes it sound more specialized than something like 頭 or 歯.

Add です:

• この神経が痛いです。

That is the polite version.

You might also hear even more natural polite speech in real conversation, depending on context, but the basic plain/polite contrast is:

• plain: この神経が痛い。
• polite: この神経が痛いです。