Modal verbs usually accompany another verb in a sentence. They not only modify the main verb, they change the nature of the sentence. Modal verbs describe the subject in relation to the action expressed by the main verb.

The modal verbs "möchten", "können" and "müssen" have the following basic meanings:

"möchten" - would like - to indicate a wish or desire:
Harry möchte in die Stadt fahren.
= Harry would like to go to the city.

"können" - can, be able - to describe an ability or option:
Harry kann Deutsch sprechen.
= Harry can speak German.
Sie können gerne ein anderes Zimmer haben.
= Harry has the option of getting a different room.

"müssen" - must, have to - indicates a necessity:
Harry muss in die Stadt fahren.
= It is absolutely necessary for Harry to get to the city.

Placement of modal verbs in a sentence
In a sentence, the modal verb is conjugated, meaning its form is an indication of the person, number and tense. It is placed in the usual position for conjugated verbs: 2nd position in simple declarative sentences and "W" questions. The main verb is placed at the end of the sentence in its basic form, the infinitive.
 

Conjugation of modal verbs
The verbs "können" and "müssen" are irregular. The verbs "möchten", "können" and "müssen" are conjugated as follows:

More:

Strictly speaking, "möchten" isn't a verb; it's a verb form of "mögen". But it is used as a modal verb. "möchten" has a similar meaning to "wollen", which is another modal verb. It's simply more polite.