Learn Hardcore Japanese: nagaku hanasu to, atama ga itaku narimasu. - When I talk for a long time, my head starts to hurt.

It is not simply joining two actions like “I talked and my head hurt.”
It’s more like “Whenever I talk for a long time, my head ends up hurting.”

Why is it 長く and not 長い? I thought the adjective is 長い.

長い is the i-adjective meaning “long.”
To modify a verb (here, 話す “to talk”), i-adjectives change ～い → ～く.

長い (long) → 長く (long-ly, in a long way / for a long time)

So:

長い話 = a long story (modifies a noun)
長く話す = to talk for a long time (modifies a verb)

Formally, 長く is an adverbial form of 長い. It describes how you talk.

Could I say 長い間話す instead of 長く話す? Do they mean the same thing?

They are close in meaning, but not exactly the same.

長く話す
- Literally: talk long.
- Very common and natural.
- Emphasizes the manner/duration of the talking in a simple way.
長い間話す
- Literally: talk for a long period (of time).
- Also correct.
- Slightly more explicit about the idea of a time span.

In this sentence:

長く話すと、頭が痛くなります is the most natural, everyday phrasing.
長い間話すと、頭が痛くなります is understandable and not wrong, but sounds a bit more formal/bookish or just less common for this everyday complaint.

Why is it 頭が and not 頭は? What is が doing here?

Here が marks 頭 as the subject of the verb phrase 痛くなります.

Literally:

頭が痛くなります。
= My head becomes painful.

Why が, not は?

が tends to mark:
- The thing that experiences the state or change.
- New or specifically focused information.
は marks the topic and has a more contrastive or background function.

You could say:

頭は痛くなります。

but that tends to feel like:

“As for my head, it gets painful (as opposed to something else).”

In a simple, neutral statement like this, Japanese normally uses:

[Body part] + が + [state/change]
e.g. 頭が痛い, お腹がすく, 足が疲れた.

So 頭が痛くなります is the default, natural pattern.

What does 痛くなります literally mean? Why not just 痛いです?

Breakdown:

痛い = painful / hurts
痛く = adverbial form of 痛い
なる = to become
なります = polite present form of なる

So 痛くなります literally means:

“becomes painful” or “comes to hurt.”

Why not 痛いです?

痛いです describes a current state:
頭が痛いです。 = My head hurts (right now).
痛くなります describes a change into that state:
頭が痛くなります。 = My head ends up hurting / starts to hurt.

In the original sentence, the meaning is:

When I talk for a long time, my head ends up becoming painful.
(It changes from “not painful” to “painful” as a result.)

That’s why Japanese prefers 痛くなります instead of simple 痛いです here.

Why is the verb なります in the present tense? Shouldn’t it be past if the headache already happened?

Japanese present tense (non-past) is used for:

General truths / habits / tendencies
- 毎日コーヒーを飲みます。
- I drink coffee every day.
Future events
- 明日行きます。 = I will go tomorrow.

In:

長く話すと、頭が痛くなります。

the speaker is describing a general tendency:

Whenever I talk for a long time, my head (always) gets painful.

If you used the past tense:

長く話すと、頭が痛くなりました。

it would sound more like a specific occasion in the past (When I talked for a long time (on that occasion), my head became painful), which is not the intention here.

Can I say 長く話したら、頭が痛くなります instead of using と? What is the difference between と and たら here?

You can say:

長く話したら、頭が痛くなります。

It is grammatically fine and understandable.

Nuance difference:

～と conditional:
- Often used for natural, automatic, or always-true results.
- Feels like “whenever X happens, Y (always) follows.”
- Fits well with physical reactions, laws of nature, etc.
～たら conditional:
- Very common, flexible “when / if / after”.
- Can be used for one-time events or more subjective results.
- Does not as strongly imply an automatic rule.

So:

長く話すと、頭が痛くなります。
Emphasizes: “Any time I talk for a long time, my head naturally ends up hurting.”
長く話したら、頭が痛くなります。
More like: “If/when I talk for a long time, my head (tends to) hurt.”
Slightly less “law-like,” but very similar in everyday use.

Both are acceptable; と just matches the “automatic result” feeling a bit more strongly.

Why is the subject “I” not said? Where is 私 in this sentence?

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

The sentence:

長く話すと、頭が痛くなります。

literally just says:

When [someone] talks long, [someone’s] head becomes painful.

But in a typical context (you talking about yourself), it is naturally understood as:

When I talk for a long time, my head starts to hurt.

You could say:

私は長く話すと、頭が痛くなります。

but:

Adding 私は is only needed if you want to contrast yourself with others (e.g. "As for me, when I talk for a long time, my head hurts, but other people are fine").
In a neutral statement about your own condition, it is more natural to leave 私 out.

Is 長く話すと頭が痛くなります the only natural word order? Can I move things around?

Japanese word order is fairly flexible, but some orders sound much more natural.

Most natural here:

長く話すと、頭が痛くなります。

Other possibilities:

頭が長く話すと痛くなります。
- Grammatically possible but sounds unnatural/confusing.
- Feels like 頭が長く (a “long head”) at first glance.
頭が、長く話すと痛くなります。
- Still awkward; sounds like you’re forcing 頭 as topic-like,
  which disrupts the smooth flow.

The usual pattern for conditional → result is:

[Condition clause] + と、[Result clause].

So keeping:

長く話すと、[result]
at the front is the most natural.

Within the result clause:

頭が痛くなります has a fairly fixed order:
[subject] が + [adjective + なる].

Why does 痛い become 痛く before なります? Is that a special rule?

Yes, it follows a regular pattern for i-adjectives.

To use an i-adjective with なる (to become), you:

Change ～い → ～く and then add なる.

Examples:

高い (tall, expensive) → 高くなる (to become tall/expensive)
静か is actually a na-adjective, so it behaves differently: 静かになる

For 痛い:

痛い → 痛くなる (to become painful)

So:

頭が痛い。 = My head hurts. (state)
頭が痛くなります。 = My head becomes painful / starts to hurt. (change into that state)

This is a very common and useful pattern for expressing changes in condition.