What conjugation of 送る is used in 返事をしたつもりだったが、送れていなかった


What is the list of conjugation forms that occurred to 送る in the sentence

> 返事をしたつもりだったが、送れていなかった。




It's the past-negative-potential-teiru form of 送る. This (て)いる refers to the
continuation of state.

  * **送る** : to send
  * **送れる** : (potential) to be able to send
  * **送れている** : (potential-teiru) have been able to send
  * **送れていない** : (negative-potential-teiru) have not been able to send
  * **送れていなかった** : (past-negative-potential-teiru) had not been able to send

The sentence means "I thought I replied, but [I noticed] I hadn't sent [the
mail] (successfully)".

Related:

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  * [ている vs ていた](https://japanese.stackexchange.com/a/57195/5010)

