The 助動詞（じょどうし） (auxiliary verb) そうだ has two primary functions in Japanese. It can convey one of the following things:
伝聞（でんぶん） - Highlighting things that have been heard from other people.
様態（ようたい） - Highlighting the way that things ‘appear’, or ‘seem’.
The 様態（ようたい） nuance is covered in another lesson, so we will focus on 伝聞（でんぶん） in this grammar point. When used to repeat statements that have been heard from other people/sources, そうだ (or そうです
) will be paired with the plain form of a verb or い-Adjective
, or a noun/な-Adjective
followed by だ
I heard that this bird chirps a lot.
I heard that the next patient was in a bad accident.
I heard that the sunset that you can see from there is beautiful.
I heard that senpai will be working tomorrow.
always implies that the speaker is not 100% certain about the validity of the information that they are conveying. Due to this, it does not sound as confident as other expressions, like ようだ
In order to avoid confusion between the そうだ that is used for repeating information, and the そうだ that is used for making observations, the following differences will need to be memorized.
降（ふ）るそうだ - Unaltered verb, そうだ that is repeating information.
降（ふ）りそうだ - 連用形（れんようけい） (counjunctive form), そうだ that is used for making observations.
寒（さむ）いそうだ - Unaltered adjective, そうだ that is repeating information.
寒（さむ）そうだ - い removed, そうだ that is used for making observations.
Nouns and な-Adjectives -
綺麗（きれい）だそうだ - だ is used before そうだ. This is repeating information.
綺麗（きれい）そうだ - No だ between the な-Adjective and そうだ. This is making an observation.