Learn Hardcore Japanese: sono suupu ha atusou da kara, ki wo tukete.

-そう (appearance) means “looks/seems (from what I can see).” So 熱そう = “looks hot (to the touch), seems hot.”
It’s used when you’re inferring from appearance (e.g., steam rising), not from direct confirmation.
Form:
- i-adjectives: drop い → 熱い → 熱そう
- Irregular: いい → よさそう
- As an attributive adjective: 熱そうなスープ (“soup that looks hot”)
熱い simply means “is hot” (a statement of fact).
To say “doesn’t look hot”: 熱くなさそう.
Note: “hot (spicy)” is 辛い (からい), not 熱い.

Why is there a だ before から (熱そうだから)?

Because 熱そう behaves like a na-adjective, the copula だ is needed before the conjunction から. General rule:

i-adjective + から: 寒いから…
noun/na-adjective + だから: 静かだから…, 熱そうだから… So you say 熱そうだから, not ✕熱そうから.

Could I use が instead of は (そのスープが熱そうだから…)?

Yes: そのスープが熱そうだから、気をつけて。 is grammatical.

が in a subordinate reason clause is the neutral choice when introducing new information.
は marks a topic/contrast. そのスープは熱そうだから… implies the soup is already known or being contrasted with other things. Both are fine; nuance differs slightly.

Is その the right “that”? When would I use この or あの instead?

この = “this (near the speaker)”
その = “that (near the listener or in shared context)”
あの = “that (far from both)” If the soup is closer to the listener or already identified in the conversation, そのスープ is natural. If it’s right by the speaker, このスープ; if it’s over there, あのスープ.

How do I make the sentence more polite?

Polite: そのスープは熱そうですから、気をつけてください。
Extra polite: そのスープは熱そうですので、お気をつけください。 Casual softener: add ね → …気をつけてね。

What does 気をつけて literally mean, and why does it use を?

気をつけて is the te-form/imperative of 気をつける “to be careful; to pay attention.”
Literally “attach/put (your) attention/awareness,” where 気 (“attention/awareness”) is the direct object, so it takes を.
The thing you’re careful about takes に: 車に気をつけて (“watch out for cars”).

Should I say そのスープに気をつけて to specify what to be careful about?

You can: そのスープに気をつけて means “Be careful with that soup.” In the original, the target is understood from context. To avoid repeating, you could say:

そのスープ、熱そうだから、気をつけて。
Or use a pronoun: それに気をつけて。

What’s the difference between から and ので here?

Both mean “because,” but:

から is more direct and common in everyday speech: …熱そうだから、気をつけて。
ので is softer/more formal or objective: …熱そうですので、気をつけてください。 In casual advice, から fits perfectly.

Can I drop anything in casual speech?

Yes, very natural:

そのスープ、熱そうだから、気をつけて。 (drop は)
You can also shorten the second part: …気をつけてね。 Japanese often omits particles when the meaning is clear.

Any spelling or word-choice pitfalls (熱い vs 暑い, etc.)?

熱い = hot to the touch/temperature of objects or food (correct here).
暑い = hot weather/air temperature.
辛い (からい) = spicy.
気をつけて can be written 気を付けて (付ける “attach”); both kana-only and mixed kanji are common.

Isn’t there another そうだ that means “I heard that…”? How do I tell them apart?

Yes, there are two:

Appearance そうだ (what we have): attaches to adjective/verb stems → 熱そうだ (“looks hot”).
Hearsay そうだ (reported speech): attaches to the full plain form → 熱いそうだ (“I hear it’s hot”). Form tells you which one it is.

Could I say 熱そうに見える instead? Any nuance difference?

熱そうだ = seems hot (general inference, often from multiple cues; concise and common).
熱そうに見える = “looks hot to the eye,” explicitly visual; slightly more tentative or descriptive. Both are fine: そのスープは熱そうに見えるから、気をつけて。

Why are there spaces between the words? Japanese usually doesn’t use them, right?

Correct. Spaces are often added in learning materials to show word boundaries. In normal writing you’d see: そのスープは熱そうだから、気をつけて。