ようだ

1. 1It seems that…; a conjecture the speaker draws from their own observation or sensory evidence.Ex. 1
2. 2It seems that…; an inference backed by visible evidence the speaker points to.Ex. 2
3. 3Like / as if; describes something by comparing it to something it resembles.

Used when the speaker draws a conclusion from their own observation, often with the evidence stated alongside, as in example 2. It is fairly neutral and suits both speech and writing. In casual conversation みたいだ is the more common equivalent, while らしい leans on information heard from others.

• らしいN4「らしい」 leans on hearsay or outside information and can mean 'I heard,' with a sense of distance, while 「ようだ」 relies on one's own observation and feelings and can also express metaphor. 「らしい」 attaches directly to nouns and na-adjective stems, without 「の」 or 「な」.
• みたいだN4「みたいだ」 has basically the same conjecture and metaphor meanings as 「ようだ」 and the two are interchangeable, but 「みたいだ」 is casual and mostly spoken, unsuitable for formal settings. It attaches directly to nouns (e.g. 「学生みたい」) and to na-adjective stems, without 「の」 or 「な」.
• そうに / そうなN4「そうに／そうな」 judges only by what something looks like right before your eyes; it attaches to adjective stems or the ます-stem of verbs, not to nouns, and with verbs it can also mean 'about to...'. 「ようだ」 attaches to the plain form, can also infer from feelings or information, and can express metaphor.
• そうだN4「そうだ」 relays information you heard, with no judgment of your own, while 「ようだ」 makes a conjecture from observation, feelings, or information at hand. With nouns and na-adjectives it takes the form 「だそうだ」, without 「の」 or 「な」. They are sometimes interchangeable, but 「ようだ」 carries the nuance of inference.
• に違いないN3They are close only in the conjecture sense: 「に違いない」 is a strong, confident assertion, while 「ようだ」 is a soft conjecture, so the tones differ and they should not be swapped. 「に違いない」 takes no 「の」 or 「な」 when attaching to nouns or na-adjective stems.
• ではあるまいかN1「ではあるまいか」 is close only in the conjecture sense: it tactfully raises an opinion or doubt in negative form, can also attach to 「べき」 to state a claim, and is strongly literary and mostly written. 「ようだ」 centers on inference from observation, is stylistically neutral, and also has a metaphor meaning.

ようだ marks the speaker's own evidence-based inference: みたいだ is its colloquial counterpart (attaching directly to nouns), らしい relies more on hearsay or outside information, そうだ and そうな/そうに describe how something looks at a glance (or, with plain forms, report hearsay). に違いない expresses far stronger certainty, and ではあるまいか is a formal, literary tentative assertion.