そうだ is one of the 18 助動詞（じょどうし） (auxiliary verbs) in Japanese, and can be used in the same way as
な-Adjectives (may be followed by な when acting as an adjective, or に when acting as an adverb). This is a unique characteristic of
そうだ and
ようだ. It is not something that can be done with all auxiliary verbs. As with
そうだ,
そうに and
そうな carry the meaning of ‘seeming like (A)’, or ‘looking like (A)’.
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さっき、先輩（せんぱい）が怒（おこ）りそうになってた。
Earlier, it seemed like senpai was going to get angry.
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彼（かれ）は忙（いそが）しそうに仕事（しごと）をしている。
He is working like he is busy.
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彼女（かのじょ）はどんな仕事（しごと）でも簡単（かんたん）そうにやるからうらやましいです。
She does all her work like it is easy, so I am very jealous of her.
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つまらなそうなパーティーには行（い）きません。
I don't go to parties that seem like they would be boring.
そう may also be used when attached to the
い-Adjective, ない. In this case, there is a unique conjugation rule which you will need to be careful of. As always, い will be removed from ない before attaching
そう. However, さ will need to be inserted between な, and
そう. This results in the construction なさそう.
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すごい！辛（から）くなさそうに食（た）べるね。
Wow! You eat it like it’s not spicy.
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大事（だいじ）じゃなさそうな物（もの）は捨（す）ててもいいよ。
You can throw away anything that does not look important.
Note that this addition of さ does not happen with verbs. This is due to the ない that is used with negative verbs being an auxiliary verb, and not an
い-Adjective. With the auxiliary verb ない, simply remove the い, before adding
そう directly to the stem.
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あの人（ひと）は怒（おこ）らなそうな顔（かお）をしているけどいつも怒（おこ）っている。
He does not look like he gets angry alot, but actually he is always angry.
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彼（かれ）は美味（おい）しくなさそうにご飯（はん）を食（た）べている。
He is eating his food like it does not taste good.