To express the desire “to want” we take the stem of the verb (i.e. the present polite form with ます cut off) and add たい. The verb now behaves like an i-adjective and conjugates in exactly the same manner.
Verb
Group
Dictionary
Stem
Want
to write
1
書く
書き〜
書きたい
to read
1
読む
読み〜
読みたい
to say
1
言う
言い〜
言いたい
to see
2
見る
見〜
見たい
to do
3
する
し〜
したい
And because we have effectively changed the verb into an i-adjective, we should use が instead of the object particle を for transitive verbs.
ビールが飲みたいです。
I want to drink beer.
それがしたかったですよ。
I wanted to do that.
That being said, you will often see the object particle used in both conversation and writing—as it was for the potential form of the verb.
景色がとてもきれいですよね。写真を撮りたいです。
The scenery is incredibly beautiful, isn’t it? I want to take a picture.
残念ですね!一緒に行きたかったです。
That’s a shame! I wanted to go together.
上司と出張に行きたくないです。
I don’t want to go on a business trip with my boss.
今夜、六本木で遊びたいです。
I want to hang out in Roppongi tonight.
To say want with a noun (e.g. “I want water”) we use the i-adjective ほしい.
新しい時計が欲しいす。
I want a new watch.
飲み物が欲しい。
I want a drink.
However, using ほしい is very direct—a child would use it to say, “I want water” or “I want something to eat”. Consequently, it is not appropriate to use to order in a restaurant, for example. We will see more polite ways of asking for something later when honorifics are introduced.