そうだ

Expresses hearsay: the speaker is passing on information heard or read from another source, 'I hear that…' or 'they say that…'. The source is often stated with によると or では.

Neutral in register; そうです is common in polite conversation, while そうだ appears in plain speech and writing. Typical contexts are relaying weather forecasts, news, and things others have said, often with a source marked by によると or では. Unlike らしい, it presents the information as straightforwardly reported rather than as the speaker's inference.

• そうに / そうなN4「そうに／そうな」comes from the appearance-use 「そうだ」and means 'looks like…,' attaching to ます-stems or adjective stems. The hearsay 「そうだ」attaches to plain forms to mean 'I heard that…' and has no 「そうな」or 「そうに」forms, so the two cannot be interchanged.
• らしいN4「らしい」is somewhat more indirect when relaying information; it can also make conjectures from signs (「風邪をひいたらしい」) and function as a suffix (「男らしい」). No 「だ」is added after nouns (「雨らしい」), and it can itself change to 「らしかった」.
• ということだN3The hearsay meaning of 「ということだ」is close to 「そうだ」and the two are often interchangeable, but its tone is more objective and formal/written; after nouns and な-adjectives, 「だ」can be omitted in the non-past affirmative. It also has an 'in other words' concluding sense that 「そうだ」lacks.
• んだってN3「んだって」can be swapped with 「そうだ」in its hearsay sense, but it is very casual — used mainly between friends and family, and not appropriate to say to your boss. Nouns and な-adjectives take 「なんだって」, and with rising intonation 「〜んだって？」can also be used to confirm something with the listener.
• によると / によればN3「によると」attaches to nouns to identify the source of the information, while 「そうだ」attaches to plain forms at the end of the sentence to express hearsay; the two cannot replace each other and are often used together. 「によると」can also introduce inferences based on maps or reference materials, paired with sentence-final expressions such as 「はずだ」and 「ようだ」.
• ってN4「って」is close to 「そうだ」only when relaying hearsay — both attach to plain forms and can end a sentence directly to mean 'I heard that…', but 「って」is casual spoken language, so use 「そうだ」in formal situations. 「って」can also mean 'called' or introduce a topic.

そうに / そうな are forms of the appearance-based そう attached to stems ('looks like'), not hearsay. らしい mixes hearsay with the speaker's own inference, ということだ is a more formal hearsay expression, んだって and って are casual spoken equivalents, and によると / によれば only marks the source and is often combined with そうだ.