How to tell them apart
ろくに〜ない judges the quality or adequacy of an action — the subject does it, but poorly or insufficiently (ろくに勉強もしない). 滅多に〜ない judges frequency — the event almost never happens at all (滅多に起きない). Swapping them is wrong because a rare event isn't necessarily done poorly, and a poorly-done action isn't necessarily rare.
- ろくにない Use ろくに〜ない when someone does an action but does it poorly, inadequately, or half-heartedly.
- 滅多にない Use 滅多に〜ない when something happens so rarely that it is almost never seen or experienced.
| Grammar point | Meaning | JLPT level |
|---|---|---|
| ろくにない rokuninai | scarcely; insufficiently; to only a small degree; almost not at all | N3 |
| 滅多にない ninai | Infrequently; on rare occasions; almost never | N1 |
ろくにない rokuninai
scarcely; insufficiently; to only a small degree; almost not at all
ろくに pairs with a negative verb to say an action is hardly done or not done well enough. It is placed directly before verbs like ろくに知らない or ろくに眠れない, including past negatives such as ろくに食べなかった. It cannot be used with an affirmative verb and often sounds critical, expressing dissatisfaction that someone did not do something sufficiently.
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この若者は、自分の国についてろくに知らない。
This young man knows little about his country.
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彼女はろくに食べないで席を立った。
She had only eaten a little before she left the table.
滅多にない ninai
Infrequently; on rare occasions; almost never
滅多に means seldom or hardly ever and always pairs with a negative verb, such as 行かない or 降らない. Place 滅多に before the verb phrase, and make the verb itself negative through ない, なかった, or ません. The negative verb does not have to end the sentence and can be followed by other endings or clauses. Avoid pairing it with negative i-adjectives, since the pattern describes rare actions or events rather than states.
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彼の機嫌がいいことなど滅多にない。
He is rarely in a good mood.
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彼は本当に嫌いな事は滅多にやらない。
He seldom does anything he really hates to do.
ろくにない and 滅多にない are JLPT N3–N1 grammar points that Japanese learners often mix up. This page compares them side by side on meanings, usage and example sentences, so you can tell them apart.