Carrying Something Through to the End
切る attached to a verb stem means the action was carried out completely.
- 長い本を読み**切った**。(I read the whole long book through.)
It is not about cutting. The literal sense survives only in the image of finishing something off cleanly.
The Three Forms
| Form | Meaning |
|---|---|
| 読み**切る** | read it all the way through |
| 読み**切れる** | be able to finish reading |
| 読み**切れない** | can't get through it all |
切れない is by far the most common, usually because the quantity is overwhelming: 数え切れない (countless), 食べ切れない (more than I can eat).
Against てしまう
Both suggest completion, and they are not the same:
| Pattern | Sense |
|---|---|
| てしまう | finished — often with regret |
| 切る | finished — with effort, all the way through |
食べてしまった can mean "I ate it, unfortunately". 食べ切った means "I got through all of it".
The Emotional Sense
With feeling verbs, 切る means doing something with full conviction: 言い切る (to state flatly), 信じ切る (to trust completely), 疲れ切る (to be utterly exhausted).