わかんない・
In casual speech, ら before ない and る before の often come out as ん: わからない becomes わかんない, and 何してるの becomes 何してんの. The meaning doesn't change.
Sorry, I don't know.
What are you doing now?
ら before ない
らない → んない
わからない → わかんない, つまらない → つまんない, 帰らない → 帰んない. The ending still changes as ない does: わかんなかった, わかんなくて, 帰んなきゃ.
る before の
るの → んの
してるの → してんの, するの → すんの, あるの → あんの. Before んだ the る just drops: してるんだ → してんだ.
れ in られない
られない → らんない
信じられない → 信じらんない, 食べられない → 食べらんない.
わかんない, つまんない: ら becomes ん
Any verb whose negative ends in らない can do this, and so can つまらない, "boring".
I don't get this question at all.
This film is boring, isn't it?
I have to hand it in by tomorrow.
出さなきゃならない: the ならない of "have to" does it too.
何してんの: る becomes ん
In conversation ている has usually lost its い already: してる. Before の, that る often becomes ん.
What are you talking about?
Are you still up?
What are you going to do now?
どうするの. It isn't only てる: any verb's る can do it before の.
何してんだ: before んだ
As a question, 何してんだ is blunter than 何してんの. You will hear it mostly from men, and often as a telling-off.
What do you think you're doing?
Oh, you already know.
Not a question, and not blunt: anyone says this.
信じらんない, 足んない: れ and り
れ and り do it too, mostly in られない, くれない, かもしれない and 足りない.
信じらんないI can't believe it
No way. I can't believe it.
信じられない.
やってらんないI can't go on like this
Overtime every day. I can't take any more of this.
やっていられない, by way of やってられない.
足んないnot enough
There's nowhere near enough time.
足りない.
てくんない？would you…?
Give me a hand, would you?
てくれない.
Who says it, and where
Friends and family talk like this all day, and type it in messages. It is not for a customer, a boss or a teacher, or for anything written with care. The full form is always right, so recognising these comes first; saying them can wait.
I'm afraid I'm not sure about that.
To a teacher, a customer or your boss.
わからん, and other short forms
わからん・知らんN2
It means the same as わかんない, but this ん is the negative itself: an old ぬ, in place of ない. Everyday speech in western Japan, and blunt talk elsewhere.
I don't know anything about that.
Check yourself
1What does this mean?
まだ
Choose an answer to see why.
2つまんなかった. What is it in full?
Choose an answer to see why.
3You are writing to your teacher. Which one fits?
すみません、
Choose an answer to see why.
More detail
触んな
The rough な of "don't" does the same to る: 触るな → 触んな, ふざけるな → ふざけんな, "stop messing around". Rough talk, common in films and anime.
Don't touch it without asking.
そりゃ, こりゃ: それは and これは
Anyone says そりゃ. こりゃ is mostly for exclaiming.
Well, of course.
Well, that's impossible.