Verb stem + たい です
To say 'I want to (do something)' in polite Japanese, take a verb's ます-stem (drop the ます), tack on たい, and finish with です: たべます → たべたい です = 'I want to eat.' That's the whole pattern. The good news is that 〜たい then conjugates exactly like an い-adjective — so the negative, past, and past-negative forms are shapes you already know.
From ます-form to 〜たい
Step 1: take the polite 〜ます form. Step 2: drop ます to get the stem. Step 3: add たい です. たべます → たべ → たべたい です ('I want to eat'). のみます → のみ → のみたい です ('I want to drink'). 行きます → 行き → 行きたい です ('I want to go'). 見ます → 見たい です. します → したい です. 来ます → 来たい です ('I want to come'). One pattern, every verb.
Negative, past, past-negative — like an い-adjective
〜たい ends in い and behaves like an い-adjective from lesson 6. Drop the final い and add くない for the negative — たべたくない です ('I don't want to eat'). For the past, drop the い and add かった — たべたかった です ('I wanted to eat'). For past-negative, たべたくなかった です ('I didn't want to eat'). The polite です at the end never changes. きのう、いえ に いたかった です が、しごと を しました。 = 'Yesterday I wanted to stay home, but I worked.'
Object: が or を
Here's a quirk: with 〜たい, the direct object can take が instead of を, especially when the want is strong or contrastive. みず が のみたい です。 = 'I want water (specifically).' みず を のみたい です。 is also fine and more neutral. With long objects or chained verbs, を is the safer default. Both are correct — が sounds a touch more emphatic / personal, and you'll hear it everywhere in everyday speech.
Asking what someone wants
Just add か at the end. なに が たべたい です か。 = 'What do you want to eat?' どこ に 行きたい です か。 = 'Where do you want to go?' なに も したくない です。 = 'I don't want to do anything.' Cultural note: 〜たい です describes your own wants. Asking a stranger or a superior 〜たい です か can feel a bit blunt; in formal settings, the pattern shifts to 〜たい です か with care, or to softer alternatives. With friends and family, it's perfectly normal.
What 〜たい can't do
〜たい is for your own wishes — what 'I' want to do (or what 'you' want, in a question). For 'he/she wants to…' Japanese uses a different ending (〜たがって います) — save it for later. For 'I want a thing' (no verb), the pattern is ほしい です: あたらしい くるま が ほしい です。 = 'I want a new car.' ほしい also works like an い-adjective. Today, focus on the 〜たい verb pattern; ほしい is in the vocab so you've seen it once.
