What's the rule for expressing "I don't want to"


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?




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)




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)

