In short

To say you want to do something, hang たい on the ます-stem (飲む -> 飲みたいです nomitai desu, “I want to drink”); to say you want a thing, use Nがほしいです N ga hoshii desu. Both behave like い-adjectives, so the negative is たくないです / ほしくないです, never ×たいじゃないです. Both are for your own desires only - for other people you switch to 〜たがっています / ほしがっています. To invite, use the negative question 〜ませんか, and to propose, 〜ましょう.

The rule

Wanting to do something: ます-stem + たい

Step 1 - Make the ます-stem exactly as you did for ます: 飲む -> 飲み, 食べる -> 食べ, する -> し, 来る -> 来 (ki).

Step 2 - Attach たい instead of ます. The pattern is ます-stem + たい: 飲みたい nomitai, 食べたい tabetai, したい shitai, 来たい kitai.

Step 3 - Inflect it as an い-adjective. This is the point learners miss. 〜たい is no longer a verb; it is an い-adjective ending in 〜い, so it takes the same four cells as 高い: たいです / たくないです / たかったです / たくなかったです. To stay polite you simply add です - the adjective itself carries the tense.

Object marking: が or を. With 〜たい the object of the original verb may keep を or switch to が. 水が飲みたいです Mizu ga nomitai desu and 水を飲みたいです Mizu o nomitai desu are both correct, and both have been standard for centuries. The が version pushes the thing into focus - it is the classic “it’s water I want to drink”, the stronger, more emotional reading. を keeps the sentence feeling like an ordinary action, and it is the safer choice whenever other words come between the object and 〜たい.

Wanting a thing: Nがほしい

ほしい hoshii (“wanted, desirable”) is a plain い-adjective, not a verb, and the thing wanted is its subject, marked が: 新しい車がほしいです Atarashii kuruma ga hoshii desu (“I want a new car”). Because it is an adjective, ×ほしいます does not exist, and the negative is ほしくないです hoshikunai desu. Note the split of labour: ほしい for a noun, 〜たい for an action.

The third-person restriction

Japanese treats another person’s inner feelings as something you cannot assert directly. So 〜たいです and ほしいです work for I (statement) and for you (question), but a flat statement about a third party needs a repair. There are three:

• 〜たがっている / ほしがっている - “shows signs of wanting”. The suffix がる turns the adjective into a verb, so it takes the 〜ている form, and - importantly - the object goes back to を: 田中さんは車をほしがっています。(〜ている itself is the next lesson’s topic; here just learn the shape.)
• 〜たいそうです / ほしいそうです - hearsay: you were told.
• 〜たいようです / ほしいようです - inference: it looks that way to you.

Question forms escape the restriction entirely: 何が食べたいですか Nani ga tabetai desu ka is perfectly normal, because you are asking, not asserting.

Invitations, proposals and offers

All three are built on the ます-stem, and they differ only in the ending.

• ます-stem + ませんか - the invitation. Counter-intuitively the negative question is the polite one: asking negatively leaves the listener room to say no, exactly like English “Won’t you join us?”. 一緒に行きませんか Issho ni ikimasen ka.
• ます-stem + ましょう - “let’s”. This assumes the other person is already on board, so it fits after agreement, or between people who have a shared plan. 行きましょう ikimashō.
• ます-stem + ましょうか - adding か makes it a genuine question, and it reads two ways. If only you act and the listener benefits, it is “shall I?” - an offer of help: 手伝いましょうか Tetsudaimashō ka (“Shall I help?”). If the action is joint, it is “shall we?”: 何を食べましょうか Nani o tabemashō ka (“What shall we eat?”).

一緒に issho ni (“together”) is the standard companion adverb for invitations and proposals.

Answering. Accept with いいですね Ii desu ne (“sounds good”) or the enthusiastic ぜひ zehi (“by all means”). Declining is done by trailing off, not by saying no: ちょっと…… chotto… (“it’s a bit…”) is a complete, understood refusal, and また今度 mata kondo (“another time”) softens it further. A blunt いいえ to an invitation lands as rude.

Forms & patterns

Building 〜たい from each verb group.

Both words inflect as い-adjectives.

Who is doing the wanting.

Inviting, proposing, offering.

Answering an invitation.

Examples

Common mistakes

• 私はパンをほしいです。 (Watashi wa pan o hoshii desu.) -> 私はパンがほしいです。 (Watashi wa pan ga hoshii desu.) - ほしい is an adjective, not a verb, so the thing wanted is its subject and takes が.
• 飲みたいじゃないです。 (Nomitai ja nai desu.) -> 飲みたくないです。 (Nomitakunai desu.) - 〜たい inflects as an い-adjective: the い becomes くない. じゃないです belongs to nouns and な-adjectives.
• 妹は自転車がほしがっています。 (Imōto wa jitensha ga hoshigatte imasu.) -> 妹は自転車をほしがっています。 (Imōto wa jitensha o hoshigatte imasu.) - がる turns the adjective into a verb, and a verb takes を. The が only survives while the word is still an adjective.
• 田中さんはビールが飲みたいです。 (Tanaka-san wa bīru ga nomitai desu.) -> 田中さんはビールを飲みたがっています。 (Tanaka-san wa bīru o nomitagatte imasu.) - you cannot assert a third person’s inner desire with bare 〜たい; use 〜たがっている, or report it with 〜たいそうです.
• 一緒に行きますか。 (Issho ni ikimasu ka.) -> 一緒に行きませんか。 (Issho ni ikimasen ka.) - the affirmative question only asks about a fact (“are you going?”). The invitation is the negative one.

Tips

• Sort by part of speech, not by meaning: ほしい and 〜たい look like verbs in English (“want”) but are adjectives in Japanese. Once that clicks, every negative, past and question form follows from lesson 8 with nothing new to learn.
• Do not ask a customer, a client or your boss 〜たいですか. Probing someone’s desires directly is intrusive in Japanese; offer instead with 〜ませんか or いかがですか ikaga desu ka (“how about it?”).
• Mnemonic for the invitation: the more negative, the more polite. 飲みますか is a bare question, 飲みませんか is an invitation, and the same logic reappears later in requests. Negation buys the listener an exit.
• ちょっと…… is not vagueness, it is the refusal. Trail off, drop your tone, and stop - the listener has already understood, and finishing the sentence with a reason often sounds less polite, not more.