Learn Hardcore Japanese: souziki wo kakenai to, heya no kaapetto ga sugu ni yogoremasu. - If I don’t use the vacuum cleaner, the carpet in the room quickly gets dirty.

The verb かける has many meanings like “to hang,” “to apply,” “to put on,” “to make (a phone call),” etc. In this expression, the idea is roughly “to run/apply the vacuum over (the floor, carpet, etc.)”.

Similar patterns:

アイロンをかける – to iron (clothes)
クリームを顔にかける – to put/apply cream on your face
ブラシをかける – to brush (e.g., hair, clothes)

You could say 掃除機を使う (“use a vacuum cleaner”), but when you mean “vacuum the carpet/floor,” 掃除機をかける sounds the most natural.

What exactly does 〜ないと mean here in 掃除機をかけないと? Is something left out?

In this sentence, 〜ないと is a shortened form of a conditional pattern:

(掃除機を) かけないと、部屋のカーペットがすぐに汚れます。
→ “If (you) don’t vacuum, the carpet gets dirty quickly.”

Here ないと is literally “if (you) don’t …”, with と as the conditional particle (“if/when”).

There is another, related use where 〜ないと is short for:

〜ないといけない / 〜ないとだめだ / 〜ないと困る
→ “have to ~ / must ~”

Example:

早く寝ないと。
(short for 早く寝ないといけない.)
→ “I have to go to bed early.”

In your sentence, though, because it is followed by a consequence (カーペットが汚れます), ないと is understood simply as “if you don’t …”, not “you must …”.

What is the role of と in かけないと? How is this different from using 〜たら or 〜なら?

Here と is the conditional particle meaning roughly “when / if”:

掃除機をかけないと、部屋のカーペットがすぐに汚れます。
→ “If/when you don’t vacuum, the carpet gets dirty quickly.”

Nuance of conditionals:

〜と
- Often used for natural, automatic, or habitual results.
- Sounds like “whenever A happens, B naturally follows.”
- Fits well with sentences like “If you press this button, the door opens,” or “If you don’t vacuum, the carpet gets dirty (as a general rule).”
〜たら (かけなかったら)
- More general “if/when” for one-time situations or hypotheticals.
- 掃除機をかけなかったら、部屋のカーペットがすぐに汚れます。
  Sounds a bit more like “If on that occasion you don’t vacuum, the carpet will get dirty.”

So ないと here emphasizes a regular cause-and-effect relationship, which fits the idea of carpets generally getting dirty if you don’t vacuum.

Why is it すぐに and not just すぐ? What does に do here?

すぐ means “immediately / soon / quickly.” To use it as an adverb directly before a verb, you can either:

Use it as-is: すぐ汚れます
Or add に to mark it more clearly as an adverb: すぐに汚れます

Both are grammatically correct here. Differences:

すぐ
- Common in spoken language.
- Slightly more casual/colloquial.
すぐに
- Feels a bit more neutral or formal.
- The に explicitly marks it as an adverbial (“in a soon way”).

Meaning-wise, すぐ汚れます and すぐに汚れます are essentially the same in this sentence.

Why is カーペット marked with が and not を? Can’t I say カーペットを汚れます?

You cannot say カーペットを汚れます. The key point is the verb type:

汚れる (よごれる) is an intransitive verb:
→ “to get/become dirty” (the subject undergoes a change)
汚す (よごす) is the transitive counterpart:
→ “to dirty / to make something dirty” (you act on an object)

In your sentence:

カーペットが汚れます。
→ “The carpet gets dirty.”

Here the carpet itself is changing state, so it must be the subject, marked with が.

If someone makes the carpet dirty, you’d use the transitive verb:

子どもがカーペットを汚しました。
→ “The child made the carpet dirty.”

So:

カーペットが汚れます – The carpet gets dirty.
カーペットを汚します – (Someone) dirties the carpet.

What is the nuance of using が for 部屋のカーペットが instead of は?

Both が and は are possible, but they have different nuances.

部屋のカーペットがすぐに汚れます。
- が marks the grammatical subject.
- Neutral, factual statement: “The carpet in the room gets dirty quickly.”
- Focus is on what gets dirty as the subject of the verb.
部屋のカーペットはすぐに汚れます。
- は marks the topic (“as for the room’s carpet…”).
- Implies contrast or emphasis:
  “As for the carpet in the room, it gets dirty quickly (as opposed to other things / in general).”

In isolation, the が version sounds like a simple description of what happens. The は version sounds more like you’re emphasizing that carpet in particular—e.g., in a conversation comparing different parts of the house.

What does 部屋のカーペット literally mean? Is の just possession like “the room’s carpet”?

Yes, 部屋のカーペット literally is “the room’s carpet,” but の doesn’t only mark strict ownership; it marks a general association.

Here it naturally means:

“the carpet in the room”
or
“the carpet of the room”

So:

部屋のカーペット – the carpet in the room
リビングのカーペット – the carpet in the living room
会社の人 – a person from my company / coworker (not necessarily “owned” by the company)

So の is not limited to possession; it often corresponds to “of / in / from” depending on context.

Who is the subject of 掃除機をかけないと? There is no “I” or “you” in the sentence.

Japanese often omits the subject when it’s clear from context or when the statement is general.

In:

掃除機をかけないと、部屋のカーペットがすぐに汚れます。

the subject of かけない is understood as something like:

(あなたが) – “you (don’t vacuum)”
or more generically (人が) – “people (don’t vacuum)”

So the natural English translation would be:

“If you don’t vacuum, the carpet in the room gets dirty quickly.”
or
“If you don’t use the vacuum cleaner, the carpet in the room will get dirty quickly.”

Japanese leaves that subject implicit; English usually needs to supply “you” or “one.”

Is it okay that the first part is plain form (かけない) and the second part is polite (汚れます)? Is that mixing levels of politeness?

This kind of “mix” is very common and natural in spoken Japanese.

Structure:

Clause 1: 掃除機をかけないと、 – plain negative
Clause 2: 部屋のカーペットがすぐに汚れます。 – polite (ます-form)

Because かけないと is a subordinate clause (“if you don’t vacuum”), it’s quite normal to keep it in plain form even when the main clause uses ます. The overall sentence still sounds polite enough.

A more formally consistent version is possible but sounds stiffer:

掃除機をかけませんと、部屋のカーペットがすぐに汚れます。

In everyday speech, your original sentence is more natural than this highly formal version.

Can I change the word order, like 掃除機をかけないと、すぐに部屋のカーペットが汚れます? Does that change the meaning?

Yes, you can change the word order somewhat:

掃除機をかけないと、部屋のカーペットがすぐに汚れます。
掃除機をかけないと、すぐに部屋のカーペットが汚れます。

Both are grammatically correct and the meaning is essentially the same:
“If you don’t vacuum, the carpet in the room gets dirty quickly.”

Subtle nuance:

部屋のカーペットがすぐに汚れます
- Slightly more focus on the carpet (“The carpet gets dirty quickly.”).
すぐに部屋のカーペットが汚れます
- Slightly more focus on how soon it happens (“Very soon, the carpet gets dirty.”).

In practice, they are both natural; the difference is minor.