Learn Hardcore Japanese: senpuuki wo tukaeba, reibou wo tuyoku sinakute mo heya wo suzusiku tamoteru to omoimasu. - If you use a fan, I think you can keep the room cool without turning the air conditioner up high.

扇風機を使う
→ “to use a fan”
Here, 扇風機 is what you “use”, so it takes を.
部屋を涼しく保つ
→ “to keep the room cool”
Here, 部屋 is what you “keep (in a certain state)”, so it also takes を.

A very common pattern is:

N を Aく / Nに Vする / Vしておく / 保つ
e.g. 部屋をきれいにする (“make the room clean”),
温度を一定に保つ (“keep the temperature constant”).

So 部屋を涼しく保てる literally means “can maintain the room in a cool state,” with 部屋 as the direct object of 保てる.

What exactly is 使えば? How is it formed, and how is it different from 使ったら or 使うと?

使えば is the -ば conditional of the verb 使う (“to use”).

Conjugation for a typical う-verb (godan verb):

Dictionary form: 使う
Stem for -ば: change う → え, then add ば
→ 使え + ば = 使えば

Meaning: “if (you) use (it)” / “when (you) use (it), then…”.

Nuance compared to other conditionals:

使えば
- Fairly neutral, slightly more formal/written.
- Often used for logical, general, or hypothetical conditions.
- Fits well in sentences expressing a general opinion like this one.
使ったら
- Past-form + ら; very common in conversation.
- Often used for specific situations in time (“when you’ve used it, then …”), but also works as “if”.
- 扇風機を使ったら、冷房を強くしなくても… is also natural.
使うと
- Often implies an automatic or natural result (“whenever A, then always B”).
- Stronger sense of a regular cause-effect.
- 扇風機を使うと、冷房を強くしなくても… is also understandable, but can sound a bit more like a rule or typical outcome.

In your sentence, 使えば nicely matches the “In general, if you use a fan, … I think” tone.

In 冷房を強くしなくても, what does 強くする literally mean here? Does it really mean “make the AC stronger”? Is this natural Japanese?

Literally:

冷房を強くする
→ “to make the cooling strong” / “to make the AC stronger”.

In practice, it means:

turn up the AC
make the AC blow colder / more powerfully
increase the cooling setting

Japanese commonly talks about AC strength using 強くする / 弱くする:

冷房を強くする = turn it up
冷房を弱くする = turn it down
Same with エアコンを強くする, 暖房を弱くする, etc.

So 冷房を強くしなくても is very natural and idiomatic Japanese for “even without turning the AC up” or “without making the AC work harder.”

How does しなくても work grammatically? Is it しなくて + も? What does も add?

Yes, しなくても is:

しない (negative of する)
→ しなくて (negative て-form)
- も (“even / also”)
  = しなくても

The pattern Vなくても (negative + も) means:

“even if (you) don’t V”
“even without V-ing”

So:

冷房を強くしなくても
→ “even if (you) don’t make the AC stronger”
→ “even without turning the AC up”

The も is what gives it the “even if / even without” feeling.
Without も, 〜なくて usually just connects to another phrase and doesn’t have that same conditional/“even if not” nuance by itself.

Could I say 冷房を強くしないでも instead of しなくても? Is there any difference between 〜なくても and 〜ないでも?

Both are used in real Japanese and often mean almost the same:

冷房を強くしなくても
冷房を強くしないでも

Both can mean: “even without turning the AC up.”

Nuance and usage:

〜なくても
- Slightly more standard / common in writing and neutral speech.
- Often taught first in textbooks.
〜ないでも
- A bit more colloquial / conversational in feel.
- Very commonly heard in speech.

In your sentence, 冷房を強くしなくても is the “safe textbook” choice, but 冷房を強くしないでも部屋を涼しく保てる is also natural and understandable.

What grammar pattern is 部屋を涼しく保てる? Why is it 涼しく and not 涼しい?

This uses a very common pattern:

N を Aく Vする / Vしておく / 保つ

Where:

N = thing whose state you control (here 部屋)
Aく = adverbial form of an い-adjective (here 涼しい → 涼しく)
V = verb of changing/keeping state (here 保つ / 保てる)

So:

涼しい (cool) → 涼しく (cool-ly / in a cool state)
部屋を涼しく保つ
→ “keep the room in a cool state” → “keep the room cool”

You cannot say 部屋を涼しい保つ; い-adjectives must become 〜く before a verb in this pattern:

静かにする (from 静かだ, a な-adjective)
高くする, 低く保つ, 明るくする, etc.

So 涼しく is required here as the adverbial/“state” form of 涼しい.

Why is it 保てる (potential form) and not just 保つ? What is the difference between 部屋を涼しく保てる and 部屋を涼しく保つ?

保てる is the potential form of 保つ:

保つ = to maintain / to keep (something in a certain state)
保てる = can maintain / be able to keep

Conjugation:

保つ (つ-verb) → stem 保て
- る → 保てる

Difference:

部屋を涼しく保つ
→ “keep the room cool” (a plain statement of doing it)
部屋を涼しく保てる
→ “can keep the room cool” / “are able to keep the room cool”

In your sentence, the meaning is about possibility/ability:

If you use a fan, then (you) can keep the room cool even without turning the AC up.

So 保てる is the natural choice.
扇風機を使えば…部屋を涼しく保つと思います would sound more like “I think (people) keep the room cool…”, which doesn’t match the intended idea of “it’s possible to keep it cool.”

What is the role of と before 思います? Is it a quoting particle meaning “that”? Could we leave it out?

Yes, this と is the quoting particle. It marks what you are thinking/ saying / feeling.

Structure:

[clause] と思います
→ “I think that [clause].”

In your sentence:

扇風機を使えば、冷房を強くしなくても部屋を涼しく保てる
= the content of the thought
… と思います
= “I think (that) …”

So literally:
“I think that, if you use a fan, you can keep the room cool even without turning the AC up.”

Normally, you cannot just drop と here:

✗ …保てる思います (incorrect)
○ …保てると思います (correct)

In casual spoken Japanese, people sometimes use って思います instead of と思います:

…保てるって思います。

But some kind of quote marker (と / って, etc.) is needed.

Why are 使えば, しなくても, and 保てる in plain form, while 思います is polite? Is it okay to mix plain and polite forms like this?

Yes, this is normal and correct.

General rule in modern polite Japanese:

Only the final main verb (or copula) in the sentence needs to be in a polite form (〜ます / です).
Verbs inside subordinate clauses, conditionals, relative clauses, etc., are usually in plain form.

So:

扇風機を使えば、
冷房を強くしなくても
部屋を涼しく保てる
と思います。 ← only this needs to be polite

This is considered an overall polite sentence because it ends in 思います.

You generally don’t say things like:

✗ 扇風機を使えば、冷房を強くしなくても部屋を涼しく保てますと思います。 (wrong / unnatural double politeness)

Subordinate clauses: plain.
Final clause: polite (or plain), depending on the level of formality you want.

Where is the subject “I” or “you” in this sentence? How do we know who is doing what?

Japanese frequently omits subjects when they are clear from context.

In your sentence:

There is no explicit “I” (like 私は) before 思います, but:
- By default, 思います is understood as “I think” (speaker’s thought).
There is also no explicit “you / we / people” as the subject of:
- 扇風機を使えば (if [someone] uses a fan)
- 冷房を強くしなくても (even if [someone] doesn’t turn the AC up)
- 部屋を涼しく保てる (can keep the room cool)

All of these are understood generically as “you / people / we” from context.

If you wanted to make it explicit, you could say:

私は、扇風機を使えば、冷房を強くしなくても部屋を涼しく保てると思います。

But in natural Japanese, this 私は is often omitted unless needed for contrast or clarity.

Could I use エアコン instead of 冷房? Is there any nuance difference?

Yes, you could say:

エアコンを強くしなくても部屋を涼しく保てると思います。

Both are natural, but there is a nuance difference:

冷房
- Literally “cooling” (as a function/mode).
- Refers more to the cooling operation, e.g. the cooling mode of an AC system.
エアコン
- Short for “air conditioner” (the device/system).
- Very common in everyday speech.
- You’ll often hear things like エアコンをつける / 消す / 強くする / 弱くする.

In casual conversation, エアコン might be more common; 冷房 can feel a bit more “functional” or technical (the cooling mode or system itself). Both are fine here.

Can I change the word order, for example 冷房を強くしなくても、扇風機を使えば部屋を涼しく保てると思います? How flexible is word order in this sentence?

Japanese word order is relatively flexible as long as particles stay attached to the right words and the overall structure is clear.

Your original:

扇風機を使えば、冷房を強くしなくても部屋を涼しく保てると思います。

Some possible variations:

冷房を強くしなくても、扇風機を使えば部屋を涼しく保てると思います。
扇風機を使えば、部屋を涼しく保てると思います。冷房を強くしなくても。 (split into two sentences; more spoken style)

However:

The general pattern [condition clause], [result clause] is very common:
- condition often comes first (here 扇風機を使えば),
- then the main statement.

Swapping them (especially across the comma) can change the emphasis slightly, but is still grammatical.
The important thing is to keep:

を / ば / ても / と etc. attached properly to their phrases
the final 思います at the end as the main predicate.