そうだ I heard that, It is said that

The auxiliary verb そうだ has two primary functions in Japanese. It can convey one of the following things:

Hearsay: 伝聞(でんぶん) - Highlighting things that have been heard from other people.

Appearance: 様態(ようたい) - Highlighting the way that things 'appear', or 'seem'.

The appearance nuance is covered in another lesson, so we will focus on hearsay 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.

そうだ 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.

Caution

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.

Verbs -

降(ふ)るそうだ - Unaltered verb, そうだ that is repeating information.

降(ふ)りそうだ - Conjunctive form, そうだ that is used for making observations.

い-Adjectives -

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.