Learn Hardcore Japanese: kyou ha suzusii desu kara, reibou wo yowaku simasu. - Because it is cool today, I will turn the air conditioner down.

Using が (今日が) here would sound unnatural, because you’re not contrasting today with some other specific time; you’re just introducing today as the topic of your comment.

What does から mean here, and how is it working in the sentence?

Here, から means “because / since.”

The structure is:

理由 (reason) + から、結果 (result)
今日はすずしいですから、冷房を弱くします。
→ “Because it’s cool today, I will turn down the air conditioning.”

So 今日はすずしいです is the reason, and 冷房を弱くします is the result that follows from that reason.

Why is it すずしい and not さむい or つめたい?

All three relate to temperature, but they’re used differently:

すずしい (涼しい): comfortably cool, usually for air / weather.
- e.g. 今日は涼しいです。 “It’s cool today.”
さむい (寒い): cold (unpleasant), usually for air / weather and how your body feels.
- e.g. 今日は寒いです。 “It’s (uncomfortably) cold today.”
つめたい (冷たい): cold to the touch, usually for objects, food, drinks, or people’s manner.
- e.g. 冷たい水 “cold water”

So 今日はすずしいです is like “It’s nice and cool today,” not cold enough to say さむい.

Why does すずしい take です? Isn’t it already an adjective?

Yes, すずしい is an い-adjective, and it can end a sentence all by itself in casual speech:

今日はすずしい。 (informal)

To make it polite, you usually add です after an い-adjective:

今日はすずしいです。 (polite)

So the pattern is:

い-adjective + です → polite statement.
The adjective itself doesn’t change; you just add です for politeness.

What exactly does 冷房 mean? Is it the same as エアコン?

冷房 (れいぼう) literally means “cooling” and refers to air conditioning in cooling mode.

In everyday conversation:

エアコン is the most common word for air conditioner (the device/system).
冷房 emphasizes the cooling function (as opposed to 暖房 “heating”).

In many contexts, you could say either:

冷房を弱くします。
エアコンを弱くします。
Both would be understood as “I’ll turn down the AC,” but 冷房 sounds a bit more like you are talking specifically about the cooling setting.

Why is it 冷房を弱くします and not 冷房は弱くします?

Here, を marks 冷房 as the direct object of the action 弱くする (“to make [something] weak / weaker”).

The pattern is:

Noun + を + 形容詞(い-adjective)の「く」form + する
→ “to make the noun [adjective].”

So:

冷房を弱くする = “to make the air conditioning weaker” → “to turn down the AC.”

If you used は (冷房は弱くします), you’d be making 冷房 the topic, which can be possible in some contexts, but here を is more natural because 冷房 is what you are directly acting on.

Why is it 弱くします and not 弱いします?

You cannot attach する directly to 弱い. With い-adjectives, you must change them to the adverbial form (～く) before する:

弱い → 弱く
強い → 強く

The construction [い-adjective]くする means “to make something [adjective].”

Examples:

音量を小さくします。 “I’ll make the volume lower.”
部屋を明るくします。 “I’ll make the room bright.”
冷房を弱くします。 “I’ll make the AC weaker / turn it down.”

So 弱いします is grammatically wrong; 弱くします is correct.

Could I also say 冷房を弱めます? What’s the difference?

Yes, 冷房を弱めます is also natural.

弱くします uses the pattern [adjective]くする, literally “make (it) weak,” and is very common in everyday speech.
弱めます uses the verb 弱める (“to weaken / to reduce”) directly.

Nuance:

冷房を弱くします feels very everyday and descriptive: “I’ll make the AC weaker.”
冷房を弱めます is straightforward “I’ll reduce the AC (power).”

In most casual situations they’re interchangeable in meaning.

Why is it します (non‑past) when in English it’s “I will turn down the AC”?

Japanese non‑past form (します) covers both present and future meanings.

Context tells you which is intended:

今、宿題をします。 “I’m doing my homework now.”
あとで宿題をします。 “I’ll do my homework later.”

In 今日はすずしいですから、冷房を弱くします。, the reason clause (because it’s cool today) suggests a decision or plan, so the natural English is with “will,” but Japanese still just uses します.

There’s a comma: ですから、冷房を弱くします。 Is that required? Can I change the order?

The comma (、) is not grammatically required, but it’s very common and makes the sentence easier to read. It simply separates:

Reason clause: 今日はすずしいですから
Result clause: 冷房を弱くします

You can’t normally swap the order like in English because から is attached to the reason clause:

✔ 今日はすずしいですから、冷房を弱くします。
❌ 冷房を弱くします、今日はすずしいですから。 (sounds unnatural in standard Japanese)

から comes after the reason, so the reason almost always comes first.

Who is the subject here? How do we know it means “I’ll turn down the AC”?

The subject is omitted, which is very common in Japanese. There’s no explicit “I” in the sentence.

We infer the subject from context and from what is most natural:

If you’re the one speaking and you’re the one who can control the AC, the implied subject is 私 (“I”).
→ “Because it’s cool today, I will turn down the AC.”

In a different context (e.g., a manager giving instructions to staff), it could be understood as “we” or “you (plural)”, but the Japanese sentence itself doesn’t specify; context does.

Could I say 今日はすずしいから、冷房を弱くします。 without です? Is that different?

Yes, you can say:

今日はすずしいから、冷房を弱くします。

This is more casual than 今日はすずしいですから、～.

Rough guideline:

すずしいですから → polite / neutral.
すずしいから → casual, often used with friends, family, etc.

Politeness levels in the two halves usually match, so in more formal speech you’d keep ですから, and in casual speech you’d use すずしいから (and might even drop します → する: 冷房を弱くする).