盛大 (seidai) conjugation
grand, prosperous, magnificent
盛大 (せいだい, seidai) is a na-adjective. Na-adjectives behave like nouns. The word itself never changes — you conjugate だ/です after it, and you need な to put it in front of a noun.
Conjugation table
| Form | Plain (casual) | Polite (~です) | When you use it |
|---|---|---|---|
| Dictionary / plain | 盛大だ seidaida | 盛大です seidaidesu | The form you look up. Also the casual “is ~”. |
| Negative | 盛大じゃない seidaijanai | 盛大じゃないです seidaijanaidesu | “is not ~”. |
| Past | 盛大だった seidaidatta | 盛大でした seidaideshita | “was ~”. |
| Past negative | 盛大じゃなかった seidaijanakatta | 盛大じゃなかったです seidaijanakattadesu | “was not ~”. |
| Te-form | 盛大で seidaide | — | Links to the next clause: “~ and …”. |
| Adverb | 盛大に seidaini | — | Turns it into “~ly”, describing a verb. |
| Before a noun | 盛大な〜 seidaina〜 | — | How it attaches when it modifies a noun. |
| Conditional (~ば/なら) | 盛大なら seidainara | — | “if it is ~”. |
| Conditional (~たら) | 盛大だったら seidaidattara | — | “if / when it is ~”. The everyday conditional. |
| Become (~なる) | 盛大になる seidaininaru | 盛大になります seidaininarimasu | “becomes ~” — the change-of-state form. |
| Too much (~すぎる) | 盛大すぎる seidaisugiru | 盛大すぎます seidaisugimasu | “too ~”. |
| Looks ~ (~そう) | 盛大そう seidaisou | 盛大そうです seidaisoudesu | “looks / seems ~” from appearance. |
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Why it conjugates this way
Na-adjectives behave like nouns. The word itself never changes — you conjugate だ/です after it, and you need な to put it in front of a noun.
See the full rules in the adjective conjugation guide.