To say that one wants to do something, one uses the 「〜たい」 form of the verb, for example, 「日本に 行きたい です」.
In the 「〜たい」 form, the word behaves like an adjective, using similar patterns such as 「〜くない」 for negation and 「〜かった」 for the past tense.
But what is the part of speech of the 「〜たい」 form word? Is it still a verb, albeit one that "acts" like an adjective? Or is it in fact an adjective, and if so, is there a grammatical concept that captures this "adjectivalization"?