Humble form of a verb : O { verb base { shimasu

III. Expression of the speakers and the heare's desire
A. When you want to say "I want something." say "Nani ka ga hoshii desu."
Examples
1. Kyasshukaado ga hoshii desu.
"I want a cash card."
2. Ii kutsu ga hoshii desu.
"I want a good pair of shoes."
3. Kusuri ga hoshii desu.
"I want some medicine."
4. Kono hon wa hoshiku arimasen.
"I don't want this book."
5. Kinoo kore ga hoshikatta n desu.
"I wanted this yesterday."
In "X ga hoshii desu.," X indicates what the speaker wants. The particle ga does not indicate the subject but the direct object of hoshii, an adjective.

B. When you want to say "I want to do something." say "Nani ka ga shitai desu."
Examples
1. Kono kikai ga tsukaitai desu.
"I want to use this machine."
2. Kono kutsu ga kaitai desu.
"I want to buy this pair of shoes."
3. Sakana ga tabetai desu.
"I want to eat fish."
What are common in these three examples above are :
1. all sentences end with desu, so they are polite ;
2. desu follows a word ending in -tai ; and
3. the particle ga precedes a word ending in -tai.

A word ending in -tai comes from a verb base { -tai. The suffix –tai indicates the speaker's and the hearer's desire and changes a verb into an –I adjective.
The sentence pattern for "I want to do something." is :

X ga verb base -tai desu.

In this sentence pattern also, ga indicates the direct object, but it sometimes changes to the particle o.
Examples
1. Nihongo no hon ga/o yomitai desu.
"I want to read a Japanese book."
2. Sakana wa tabetaku arimasen.
"I don't want to eat fish."
3. Kinoo depaato de kaimono ga shitakatta desu.
"I wanted to do some shopping at a department store yesterday."
C. In Japanese culture, it is not good manners to ask one's superior's desire directly.
Instead, you should say "X wa doo/ikaga desu ka." As explained in Lesson 3,
NOTES ON SENTENCE GRAMMAR IV, ikaga is the polite counterpart of doo, so ikaga is better to use when talking to your superiors.
Example
A : Sensee, koohii wa ikaga desuka.
"Would you like some coffee, professor?"
B : Ee, itadakimasu yo.
"Yes, please."