Learn Hardcore Japanese: kami ga nobite kita kara, raisyuu biyouin ni ikou to omou. - My hair has grown long, so I’m thinking of going to the hair salon next week.

髪が伸びてきた = “(my) hair has grown” – focusing on the event/state of the hair growing.
髪は伸びてきた is also possible, but は would make hair the topic, often implying contrast or known information (e.g., “As for my hair, it’s grown (unlike something else)”).

In a neutral, simple statement reporting a change, が is the default and sounds the most natural here.

What exactly does 伸びてきた mean, and how is it different from just 伸びた?

Both involve the verb 伸びる (to grow, to lengthen), but the nuance differs:

伸びた
- Simple past / result: “grew” or “has grown.”
- Focuses on the completed result.
伸びてきた = 伸びて + くる + た
- Vてくる often means “to (gradually) become ~ up until now.”
- Implies a process leading to the current state:
  - “It’s (been) growing and has now gotten (to this point).”
- In context: 髪が伸びてきた feels like “My hair has been growing and now it’s gotten long (enough that I notice / want to do something).”

So 伸びてきた emphasizes a change over time that has resulted in the current situation.

Grammatically, how is 伸びてきた formed?

Breakdown:

Dictionary form: 伸びる (to grow)
て-form: 伸びて
Add auxiliary verb くる in its plain past きた

So:

伸びる → 伸びて + くる → 伸びてくる → 伸びてきた

Here くる is not the literal “to come” toward the speaker, but the auxiliary meaning “come to be ~ / gradually become ~ up to now.”
伸びてきた = “has (come to) grow / has grown (over time, up till now).”

What is the function of から in this sentence?

から here is a conjunction meaning “because / since.”

Structure:

髪が伸びてきたから、来週美容院に行こうと思う。
Because my hair has grown, I think I’ll go to the salon next week.

The clause before から gives the reason, and the clause after から gives the result or decision.

Can I put the “because” part at the end, like in English: “I’ll go … because my hair has grown”?

Yes, in casual spoken Japanese you can say:

来週美容院に行こうと思う。髪が伸びてきたから。

This is very natural in conversation. Technically, the 〜から clause is a subordinate (dependent) clause, so putting it in a separate sentence is a bit “incomplete” in strict grammar terms, but it’s extremely common in real speech.

You would not usually say:

✕ 来週美容院に行こうと思うから、髪が伸びてきた。

That would sound like “Because I think I’ll go to the salon next week, my hair has grown,” which reverses cause and effect.

Why is に used after 美容院, and could I use へ instead?

美容院に行く uses に as the destination marker:

X に行く = “go to X.”

You could say 美容院へ行く; へ also marks direction/destination. The differences:

に
- Very common and neutral for destinations.
- Often focuses on arriving at / reaching the place.
へ
- Slightly more “directional,” like “toward.”
- More common in written style or announcements; everyday speech tends to favor に.

In this sentence, 美容院に行こう is what you’d normally say in conversation.

What form is 行こう, and how is it different from 行く / 行きます?

行こう is the volitional form of 行く (“to go”).

行く = plain non-past, neutral statement of fact: “(I) go / will go.”
行きます = polite form of 行く.
行こう = volitional form: “let’s go” / “I’ll go” / “I’m going to (decide to) go.”

Nuances:

By itself, 行こう can be:
- A suggestion: “Let’s go.”
- A self-directed decision: “OK, I’ll go / I guess I’ll go.”

In 行こうと思う, it’s expressing intention: “I think I’ll go / I’m planning to go.”

What does と do in 行こうと思う?

と here is a quotative particle. It marks what is being “said” or “thought” as a kind of quotation:

行こう is the content of the thought.
思う is “to think.”
So 行こうと思う literally is like: “I think, ‘Let’s go / I will go.’”

General pattern:

「〜」と思う = “think ‘〜’.”
「〜」と言う = “say ‘〜’.”

Whenever you see X と思う, think: I think ‘X’.

What is the difference between 行こうと思う and 行こうと思っている?

Both express intention, but nuance differs:

行こうと思う
- A decision or thought at or around the time of speaking.
- Often sounds like you’re forming or stating the intention now.
- “I think I’ll go / I’m going to go.”
行こうと思っている
- Describes a plan or intention that has been in your mind for some time and is still ongoing.
- “I’ve been thinking of going / I’m planning to go (and have been for a while).”

In your sentence, 行こうと思う sounds like:
“Since my hair has grown, I (now) think I’ll go to the salon next week.”
If you used 行こうと思っている, it would feel more like:
“I’ve been thinking of going to the salon next week (because my hair has been growing).”

Why isn’t there an explicit subject like “I” in the Japanese sentence?

Japanese often omits the subject when it’s clear from context.

Here, the subject is naturally understood to be “I” because:

髪が伸びてきた – people usually talk about their own hair unless otherwise specified.
行こうと思う – intentions and thoughts are normally about oneself unless another subject is somehow indicated.

So the full “English-like” version would be:

(私は) 髪が伸びてきたから、来週美容院に行こうと思う。
(As for me,) since my hair has grown, I think I’ll go to the salon next week.

But Japanese leaves 私は out as unnecessary.

Why is 思う in the present tense (思う) when it’s talking about a future action (going next week)?

Japanese doesn’t have a separate future tense like English. The non-past form (e.g., 思う, 行く) covers both present and future.

In this expression:

行こうと思う
- Literally: “I think ‘I will go.’”
- The thinking is in the present, and the going is in the future (next week).

So 思う is correctly in non-past form, and 来週 (“next week”) provides the future timing.

Is this sentence casual or polite? How would you say it politely?

The original sentence is in casual/plain form:

髪が伸びてきたから、来週美容院に行こうと思う。

To make it polite, you would change mainly the last verb:

行こうと思います。

So a polite version:

髪が伸びてきたので、来週美容院に行こうと思います。

You can keep から in polite speech too:

髪が伸びてきたから、来週美容院に行こうと思います。

Using 思います makes it appropriate in more formal or respectful contexts.

What is the difference between 美容院, 美容室, and 床屋?

All relate to places where you get your hair cut, but nuances differ:

美容院 (びよういん)
- Beauty salon / hair salon.
- Often associated with services like styling, coloring, perms, treatments, etc.
- Commonly used and natural in this sentence.
美容室 (びようしつ)
- Also “beauty salon.”
- More of a formal or business-like term; often seen in shop names, signs.
床屋 (とこや)
- Barbershop.
- Traditionally associated with men’s haircuts, shaves, etc.

In this sentence, 美容院 suggests going to a salon (rather than a simple barber), which fits well with thinking about hair length and style.

What does 来週 modify here? Is it tied just to 行こう, or to the whole sentence?

来週 (“next week”) is an adverbial time expression that primarily modifies the going action:

来週美容院に行こうと思う。
(I think I will go to the salon next week.)

By implication, the reason clause (髪が伸びてきたから) applies now and leads to the decision about what you will do next week.
So structurally it modifies 行こう, but semantically it sets the time for the future plan expressed by 行こうと思う.

Could I use ので instead of から here? Would the meaning change?

Yes, you can say:

髪が伸びてきたので、来週美容院に行こうと思う。

Both から and ので mean “because / since,” but the nuance is slightly different:

から
- More direct, often used in everyday conversation.
- Neutral; can sometimes feel a bit more “bare” or straightforward.
ので
- Often sounds a bit softer / more formal / more explanatory.
- Frequently used in polite speech or when you want to be less blunt.

In casual speech among friends, から is very natural. ので is also correct and sounds a touch more formal or gentle.