I've seen many texts where verbs ends in く and this remind me the transformation from i-adj to adverbs where we simply change い --> く
This is the sentence I'm struggling with: ミトがいらないなら買う必要なくね?
I understand the general meaning. I would translate it as: "If you (Mito) don't need it, it is not necessary to buy it right?". But the way I would write this in Japanese would be:
ミトがいらないなら買う必要ないでしょう？
Why ない is converted to なく? I've seen this in many other sentences.
Thank you!
It's colloquialism of 買う必要なくない？, which is double negation ("Isn't it needless to buy?"). The former ない has conjugated to the adverbial form なく to modify the latter ない.
This is actually a masculine-ish way to say 'there's no need, is there?'
It's the product of two phenomena:
First, it's common to ask questions you expect a 'yes' answer to with some sort of negation marker (different negator choices create different connotations); when you expect a 'no' answer, you do the same thing to a negative verb. Thus:
Expected answer: 必要ない
↳ Question expecting answer: 必要なくない？
This sort of negating-a-negative strategy specifically has a sense of not quite being sure the answer you expect is true, or at least seeking confirmation that it is in fact true. (This specific strategy doesn't seem to be valid for verbs and adjectives that aren't already negative; you would use like 必要あるじゃない？ or maybe 必要あるだろう？ to get the same effect.)
Second, it's a feature of colloquial masculine speech to turn /ai/ sequences into /ee/ (though mostly outside of word roots; もんだいねえ is normal but もんでえねえ is extremely unusual). This one's been truncated due to being at the end of a sentence; that happens in written colloquial speech a lot (cf でしょ？).
What you're seeing in these sentences is most likely omission. The full sentence would be:
ミトが要らないなら買う必要なくなる
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EDIT:
なくない didn't come into mind when I was answering, but thinking about it now I believe it's more likely than なくなる(although I have seen such omissions before).
