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.
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.
|I want to drink beer.|
|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.
|The scenery is incredibly beautiful, isn’t it? I want to take a picture.|
|That’s a shame! I wanted to go together.|
|I don’t want to go on a business trip with my boss.|
|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.
|I want a new watch.|
|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.