有効 (yuukou) conjugation
valid, effectual
有効 (ゆうこう, yuukou) 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 | 有効だ yuukouda | 有効です yuukoudesu | The form you look up. Also the casual “is ~”. |
| Negative | 有効じゃない yuukoujanai | 有効じゃないです yuukoujanaidesu | “is not ~”. |
| Past | 有効だった yuukoudatta | 有効でした yuukoudeshita | “was ~”. |
| Past negative | 有効じゃなかった yuukoujanakatta | 有効じゃなかったです yuukoujanakattadesu | “was not ~”. |
| Te-form | 有効で yuukoude | — | Links to the next clause: “~ and …”. |
| Adverb | 有効に yuukouni | — | Turns it into “~ly”, describing a verb. |
| Before a noun | 有効な〜 yuukouna〜 | — | How it attaches when it modifies a noun. |
| Conditional (~ば/なら) | 有効なら yuukounara | — | “if it is ~”. |
| Conditional (~たら) | 有効だったら yuukoudattara | — | “if / when it is ~”. The everyday conditional. |
| Become (~なる) | 有効になる yuukouninaru | 有効になります yuukouninarimasu | “becomes ~” — the change-of-state form. |
| Too much (~すぎる) | 有効すぎる yuukousugiru | 有効すぎます yuukousugimasu | “too ~”. |
| Looks ~ (~そう) | 有効そう yuukousou | 有効そうです yuukousoudesu | “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.