〜たい

〜たい expresses the speaker's own desire to do something — "I want to do X." 食べたい = "I want to eat." Attaches to the same i-form base as ます: 行きたい, 見たい, 来たい.

The key constraint: 〜たい is almost exclusively for the first person (the speaker's desire) or for questions directed at the second person. You cannot use 〜たい to assert what another person wants — "He wants to eat" is not 彼は食べたい. For third-person desire, use 〜たがる ("shows signs of wanting") or 〜たいと言っている ("says they want to"). This restriction reflects a wider Japanese principle: you cannot directly assert another person's internal mental state.

たい inflects as an i-adjective: 食べたい ("want to eat"), 食べたくない ("don't want to eat"), 食べたかった ("wanted to eat"), 食べたくなかった ("didn't want to eat"). Adding です to any of these gives the polite-register form: 食べたいです.

The object of a たい verb can take either を or が — 寿司を食べたい / 寿司が食べたい are both natural; が emphasizes what specifically is wanted.