When used with the verb stem そう expresses that something is likely to happen or be true based on your own first-hand experience or judgment.
雨が降りそうだ。
It looks like it’s going to rain.
Context: You’ve looked up at the sky and seen dark clouds.
時間がかかりそうだね。
It seems like it’s going to take some time, eh?
Context: You think the task is going to be difficult.
彼女に興味がありそうな顔をしているよね。
He’s got a look that says he’s interested in her, right?
Context: You’re sat across the table and your friend keeps looking at the girl.
ありそうな理由。
A likely reason.
We can connect the phrase to a noun using な and to a verb using に.
子供たちが楽しそうに遊んでいる。
The children are playing happily (looking like they are having fun).
対向車にぶつかりそうでした。
I almost hit an oncoming car.
It can also be used with the potential form.
できそうですか。
Do you reckon you can do it? (Does it seem like you can do it?)
To make a verb sentence negative we add にない (or もない).
できそうにない。
It doesn’t seem I can do it.
Using そう with Adjectives
そう can also be used with adjectives. For i-adjectives we need to get rid of the final い; for na-adjectives we append そう without the な particle.
家賃が高そうなアパート。
An expensive-looking apartment.
Context: You don’t know the rent but you’ve seen the luxury furnishings inside.
ケーキがうまそう！
The cake looks delicious!
便利そうな制度。
A convenient-sounding system.
Context: You’ve not used the system yourself, but you’ve read the details and can imagine it’s convenient.
Adjective Exceptions & Negatives (Crucial for JLPT)
There are two highly common irregulars that frequently appear as trick questions on the JLPT. For the adjective いい (good) and the negative suffix ない (not), you must change them to よさそう and なさそう.
Consequently, to make an i-adjective negative using this grammar, you conjugate the adjective into its negative form (e.g., 高くない), drop the final い, and add さそう (高くなさそう).