かな・
かな is "I wonder": a question you ask yourself out loud. Someone beside you may answer, but you aren't making them. かしら means the same; you will hear it far less, and mostly from women.
with a question word
Now, where did I put my keys?
a soft question
Would it be okay if I went home now?
How to form it
Plain form + かな／かしら
行くかな, 行ったかな, 寒いかな. A noun or a な-adjective drops だ: 休みかな, 大丈夫かな (but 休みだったかな). Drawn out, it is かなあ.
Wondering out loud
You are thinking aloud, alone or with someone. Often it is a question nobody there can answer.
I wonder if I left my wallet at home.
I wonder if this cake is still okay to eat.
I wonder if I'll make the train…
Drawn out, the worry lingers.
I wonder if this new place is good.
I wonder. Shall we try it?
B doesn't know either, and wonders along.
Asking, or wondering?
Is Tanaka coming?
A question for the other person. You think they know.
I wonder if Tanaka is coming.
A thought out loud. Nobody has to know.
Said to someone: a soft question
About something the other person can answer or decide, かな asks gently, as if you were only thinking. It is casual.
Is nine okay for meeting up tomorrow?
Yeah, nine is fine.
Would it be all right if I sat here?
To a stranger, or to someone above you.
Things people say with it
どうしようかなwhat should I do…
Hmm, what should I do about lunch…
〜（よ）うかなmaybe I'll…
Maybe I'll go to bed early tonight.
After 行こう or 寝よう: you are thinking of doing it and haven't decided.
〜ないかなI wish…; I hope…
I wish summer vacation would hurry up and come.
Usually a wish. The ない isn't "not": you want it to happen.
そうかなI'm not so sure
You think so? I like it.
A soft way to disagree.
On your own answer or opinion it is "I guess": it keeps you from sounding too sure.
About what time will you get here?
Around three, I guess.
かしら
かしら means the same and is formed the same way. Today it is far less common than かな. You will hear it mostly from women, older more than younger, and more in novels, films and anime than in real life. かな is the one anyone can say.
I wonder if that shop is open today.
Would it be all right if I opened the window a little more?
Close neighbours
でしょうかN4
The soft question in polite speech: to a teacher, a boss, a stranger, a customer.
May I use this seat?
なあN5
The same な without か: a feeling said to yourself, with no question in it.
The sky really is getting dark.
っけN3
For something you once knew and can't call to mind: "what was it again?"
Where did I put my keys again?
Check yourself
1Which one fits?
明日の
Choose an answer to see why.
2What does this mean?
早く
Choose an answer to see why.
3You ask your teacher. Which ending fits?
先生、
Choose an answer to see why.
More detail
かなと思う
With と思う after it, かな floats a plan or an opinion without committing you. People say this in polite speech too.
I'm thinking I might go to Japan next year.
Wondering in polite speech
After です or ます, people wonder aloud with かね.
I wonder if the bus will be here soon.
In writing: だろうか
Will he really come, I wonder.
か alone, said to yourself
With no な, a plain form and a falling か is a decision or a discovery, not wondering.
Right, time to go home.
Five o'clock already.
More sentences
I wonder if the exam will be difficult.
I wonder if this box will fit in the car.
I wonder if I left the key on the desk.
I wonder if he is a student.
I wonder if this explanation gets the point across.