1.
valid, in effect
Legally or officially acceptable and usable.
この切符はまだ有効だ。
This ticket is still valid.
このクーポンは来月まで有効です。
This coupon is valid until next month.
パスポートの有効期限を確認してください。
Please check the expiration date of your passport.
有効期限 = expiration date (literally: validity period)
契約は来年の三月まで有効なので、それまでに更新する必要がある。
The contract is valid until next March, so it needs to be renewed by then.
この割引カードは発行から一年間有効で、期限が切れたら自動的に無効になる。
This discount card is valid for one year from issue, and becomes invalid automatically when it expires.
無効 = invalid (opposite of 有効)
2.
effective, useful
Producing the intended or desired result.
有効な方法を考える。
To think of an effective method.
時間を有効に使いましょう。
Let's use our time effectively.
有効に使う = to use effectively
この方法は有効だと思う。
I think this method is effective.
運動は健康を保つために有効な手段の一つだ。
Exercise is one of the effective means of maintaining health.
有効な手段 = effective means
限られた資源を有効活用するために、リサイクルが大切だと言われている。
It is said that recycling is important in order to make effective use of limited resources.
有効活用 = effective utilization
USAGE: 有効 is used for both (1) validity of documents, tickets, etc., and (2) effectiveness of methods, measures, etc.
COMMON PATTERNS:
- 有効期限 (expiration date)
- 有効に使う (to use effectively)
- 有効な手段 (effective means)
- 有効活用 (effective utilization)
CONTRAST:
- 無効 - invalid, ineffective
NOTE: Do not confuse with 友好 (friendship), which has the same reading.