More questions with っけ, かな, かい, だい, じゃない, and じゃん

っけ is used when the speaker has forgotten something or is trying to recall something they once knew. It is usually, although not exclusively, used as a question directed to oneself.

なんだっけ？
What was it again?

どこに置いたっけ
Where'd I put it, again?

っけ is almost never used with non-nouns except in the past tense. When it is, other stuff (like explanatory んだ) is usually put in between.

どこにあったっけ？
Where was it, again?

こういうの何て言うんだっけ。
What was this thing called, again?

Similar to っけ, there is also かな. It is usually pronounced with a rising intonation as a question, but just like っけ it is a self-directed one. It simply means the speaker is wondering or unsure or hesitant about something, similar to "I wonder" in English. Needless to say it comes from the union of the question particle か and the sentence ending particle な.

話したいことがあるんだけど、いいかな？
I have something I need to talk to you about, I wonder if now is okay? 

It often comes after the volitional form of a verb to indicate that the speaker is about to decide to do something.

さて、本でも読もうかな
Well then, shall we read a book or something?

かい is a "friendly" question marker, especially for masculine speakers. Outside of familiar conversation, it can come across as rude/too direct. It’s not as common in real life as it is in media. In standard usage, it can only be used to ask yes/no questions.

お嬢ちゃん、大丈夫かい？
Young lady, are you alright?

Parallel to かい, there is also だい. The vibes are pretty much the same, but unlike かい, だい is used to ask questions that expect an actual answer.

君たちはこれからどうするんだい？
What are you guys going to do from now on?

じゃない can also be used to ask confirmation questions. Yes, when attached to nouns, it's identical to normal "is not". No, there's no way around it. If you are lucky, there will be a question mark after it.

いいじゃない
It's fine, isn't it?

Note how adding a copula after verbs and い adjectives is technically incorrect but this type of じゃない lives on its own and can show up after them. It’s not negating the predicate, it’s just adding a negative question nuance (“isn’t it?”) and behaves like a sentence-ending particle.

じゃん is a casual contraction of じゃない