As in English, Germans use the Subjunctive form of Modal Verbs to be more polite.

           MODELL:
                       Can you help me?      vs.  Could you help me?
                       Kannst du mir helfen?  vs.  Könntest du mir helfen?

You already know one Subjunctive form of the Modal Verbs:  möchten [derived from mögen]
 

•  The Subjunctive form of the Modal verbs is formed from the Simple Past form of the Verb
•  Add an Umlaut only if there is an umlaut in the Infinitive form of the verb.

          MODELL:
                         Könntest du mir helfen?  [können has an umlaut]
                         Wolltest du mir helfen?    [wollen has no umlaut]