～そうだ acts like a 形容動詞 and is only used with adjectives or verbs. It just can't be used with plain nouns．Its usages have significant differences in grammar. There are two grammatical ways it can be used. In this lesson, we will focus on how it is used when it attaches to the 連用形.
From the outward appearance, ～そうだ expresses the speaker's intuitive subjective guess on the nature, quality, or properties of something or someone. The statement isn't proven. So, if obvious, it's not applicable. This is like "seem to be (doing)". ～そうだ attaches to the 連用形 for verbs but the stems of adjectives for conjecture.
The negative can be ～なさそうだ or ～そうじゃない. One may be more preferable depending on the situation and or the opinion of the speaker. With adjectives in the negative and よい, さ should be inserted. So, ～なさそうだ and よさそうだ.
The new manager is a scary-looking guy, isn't he?
This video game doesn't seem interesting.
You look good today.
The weather seems nice today in Malaysia.
It doesn't look like it's going to rain.
It seems like it will continue for a little longer.
This steak looks delicious.
8. 金髪の女が好きそうだな。(Masculine; casual)
I bet you like blond women!
It looks like you can still use it.
It's a difficult looking book.
11. なんだかずいぶんよさそうじゃないか？ (Casual)
Somehow it doesn't really seem amazing does it?
He doesn't seem to be feeling painful.
The building seems quiet.
It seems that he's going to come.
He didn't look happy.
This poor dog.
Meaning Note: Notice how the meaning of かわいい changes to "poor" with ～そうだ.
漢字 Note: 可哀相 is 当て字. 可哀想 is also possible.
At first sight, the book seems easy.
18. 忙しそうです。 VS 忙しいようです。
Seems to be busy. Looks busy.
Nuance Note: As this shows, ～そうだ shows something from an intuitive judgment based on observation.You get the sense that the person is busy. Maybe he's constantly looking at his clock. ～ようだ indicates that the judgment is based on actual knowledge of the person's situation. You may have heard that he was busy, or you know information concerning his work schedule.
Kazunari reluctantly put the phone receiver down. The middle-aged security guard was giving a questioning look, so Kazunari immediately deciding to leave the area.
From 白夜行 by 東野圭吾.
Grammar Note: Remember that this structure is adjectival. So, you can get forms like ～そうな and ～そうに.
Word Note: 初老 originally meant someone in one's forties. As people now live longer, some consider it to mean mid-fifties to even early sixties.
Again, with よい and ない, さ is inserted. This gives よさそう and なさそう. As for the auxiliaries ～たい and ～ない, ～そうだ attaches to the stem. However, さ is being attached more and more. Even for adjectives that end in ない like ぎこちない (awkward), ～さ is being attached. The negative is ～そうではない, but you can also use ～な（さ）そうだ. However, something like "知らなそうだ" is correct. You still get examples like 20, though.
Naruse said in a disappointed tone.
From 顔に降りかかる雨 by 桐野夏生.
2. Shows a judgment based on circumstances or experience. The negative is そう［に・も］ない.
It would seem best to not go home at this rate.
If now, we can still be there on time.
The economy still appears that it won't improve.
3. Shows that something looks like it's going to or has...just as before. There is a basis with some experience. After all, how can you know if it's going to rain if you've never seen rain? This is an affirmative application of 1 seen with verbs. This can be seen with the past tense, and time phrases are often used to specify when something is thought to occur.
The baby had a face as if he was going to burst out crying.
For the time being, this heat wave will probably settle.
I lost my balance, and for a moment it seemed I would fall.
He is likely to live to 100.
This camera looks broken.
The lights appear to be dying.
It looks like it will be able to become better.
It looks like it's going to rain any moment.
The cherry blossom trees are about to bloom.
I feel that I can pass the exam.
It appears that war or something dangerous is going to occur in the Korean peninsula.