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。
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.
The sentence means "I thought I replied, but [I noticed] I hadn't sent [the mail] (successfully)".
Related:
This 送れて is the te-form of the "potential form" 送れる of 送る.
The potential form is sometimes used to mean that something was done successfully
メールを送れた
I (successfully) sent the email (lit. I was able to send the email)
In the sentence
返事をしたつもりだったが、送れていなかった
the te-form (which together with いる may be used to express a state) is used to express a state of "not having been sent (successfully)", so
I was under the impression that I replied [to the email], but it hadn't been successfully sent