At a glance
足りない means "not enough" or "insufficient". It is the negative of the る-verb 足りる "to be enough" and is used when something exists but not in the amount or degree needed.
[Noun] が 足りない。
時間が足りない。 There isn't enough time.
How to form it
足りる is a る-verb and is intransitive. The thing that is lacking takes が (or は when it is the topic). It conjugates like this:
| Form | Polite | Casual |
|---|---|---|
| Affirmative | 足ります | 足りる |
| Negative | 足りません | 足りない |
| Past negative | 足りませんでした | 足りなかった |
| て-form (negative) | - | 足りなくて |
An amount can come right before it: 百円足りない "100 yen short".