If what you want is to perform an action, you must modify the verb in question, rather than use the adjective ほしい. To do this you must transform the verb into an い-adjective by placing たい at the end of the stem of the verb. After the verb has been modified in this way, it behaves just like any other い-adjective when you want to conjugate it into the negative or past tense. See the examples below to see how this is done.
|I want to drink beer.||私 は ビール が 飲みたい です。watasi ha biiru ga nomitai desu.|
|(I) want to learn Japanese.||日本語 が 習いたい です。nihongo ga naraitai desu.|
|What do (you) want to eat tomorrow?||明日 は 何 が 食べたい です か？asita ha nani ga tabetai desu ka?|
|She didn't want to go to the doctor.||彼女 は 医者 に 行きたく なかった です。kanozyo ha isya ni ikitaku nakatta desu.|