is a 終助詞（しゅうじょし） (sentence ending particle) in Japanese that is used to convey when the speaker is trying to either remember, or confirm information that they have forgotten. In this way, it is very similar to the ‘again’ in ‘what is (A) again?’
may only be used after the past form of the 助動詞（じょどうし） (auxiliary verbs) だ or た. This means that け (should) only be seen as だっけ or たっけ.
Did I eat breakfast this morning?
Um, Taylor, was your car blue, again?
Was I free on Saturday, again?
Who was that again? Shoot, I can't remember.
It is also common to see のだ and んだ utilized to make verb tenses a possibility.
Kumiko, were you able to eat natto again?
You are the one editing the paper, right?
Despite the grammar rules related to っけ
, the younger generation quite often uses it without the addition of んだ (especially with verbs).
While it is important to understand that this is still っけ
, it is not a grammatically correct form that may be used in writing (or formal speaking).
け will sound equally natural when asking a rhetorical question to yourself, or when directly asking another person about something that you have forgotten.
Umm, did I lock the door? (Asking oneself)
Do you remember if we could take our dogs to that restaurant over there? (Asking another)