Questions & Answers about tosyokan ha sizuka da.
は is the topic marker. It tells us that “図書館” (the library) is what we’re talking about. In English you might think of it like saying “As for the library, (it is…)”.
- は sets or contrasts the topic.
- が marks a new subject or emphasizes it (“There is/ it is …”).
If you said 図書館が静かだ, you’d be focusing on the fact that “it is the library that is quiet,” perhaps in contrast to something else.
だ is the plain (dictionary) form of the copula, similar to the English verb “to be” (is/am/are).
- After な-adjectives (like 静か), you attach だ to make a complete predicate.
- Without だ, it would be incomplete in plain speech:
- 図書館は静か sounds unfinished (more like an adjective stuck to the topic).
In polite speech you’d use です instead of だ (図書館は静かです).
When using 静か as an adjective before a noun, you must add な (静かな図書館). But when 静か is the predicate (describing the topic after the verb “to be”), you drop the な and use the copula:
- Before noun: 静か
- な
- noun → 静かな図書館
- な
- After topic: 静か
- だ → 図書館は静かだ
Japanese follows a topic–comment (or subject–predicate) order:
- Topic/Subject → 2. Predicate (description/verb)
Here: - 図書館は (topic)
- 静かだ (predicate)
Switching them would break that basic pattern and sound ungrammatical.
In very casual spoken Japanese, you can drop だ after な-adjectives and nouns:
- Casual: 図書館は静か (sounds relaxed/informal)
- Slightly more formal/plain: 図書館は静かだ
- Polite: 図書館は静かです
Dropping だ is common among friends or in diary-style writing.
It’s a matter of politeness level:
- 静かだ → plain, informal (used in casual conversation among peers or in writing like novels)
- 静かです → polite, formal (used with strangers, elders, in business settings)
Both mean “is quiet” but fit different social situations.