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As far as I know, for expressing you want to do something, you have to just
attach たい to the verb stem like in カニを食べたい。

Then, what's the rule for saying "I don't want to". I've seen in this phrase
怖い映画は、見たくないよ , they added "たくない" to the verb stem, but is that always so, or
how is it?
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2016-08-24T22:27:13.407
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grammar verbs auxiliaries
What's the rule for expressing "I don't want to"
Yup, that's exactly right.

V-stem + たい "Want to V"

V-stem + たくない "(I) don't want to V"

[Source](http://www.learn-japanese-adventure.com/japanese-expression-
desire.html)
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2016-08-24T23:06:53.287
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Adding to @ishikun's answer, you need to note that
[たい](http://jisho.org/search/%E3%81%9F%E3%81%84) is an (auxiliary) adjective
which uses 'adjective stem (い removed from い adjective) + くない' to [negate
it](http://www.guidetojapanese.org/learn/grammar/adjectives).

I would say

> They added "くない" to the adjective stem.

rather than

> They added "たくない" to the verb stem.

You will find the linked question interesting. [In what way is the negative
form of a verb an
adjective?](https://japanese.stackexchange.com/questions/4746/in-what-way-is-
the-negative-form-of-a-verb-an-adjective/5110#5110)
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2016-08-25T07:47:54.397
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To add to the other answers, if you want to make a contrastive/emphatic
statement, you can of course add a は or も before the ない.

>   * 朝食を抜きたく **は** ないけど、今出かけないと電車に間に合わないよ。 → I don't _want_ to skip
> breakfast, but if I don't leave now I'll miss the train.
>   * 行きたく **も** 残りたく **も** ない → I don't want to stay or go.
>   * ライブを見逃したく **は** ないけど、おばあさんの面倒を見ないとだめだから、行きようがない。 → It's not that I want
> to miss the concert, but I have to take care of my grandmother, so there's
> no way I can go.
>
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