The singular forms of modal verbs are irregular because…

1.       They usually feature a stem-vowel change. [In wollen o → i]

2.       You do not add an ending onto the stem of the verb for either the ich or the er/sie/es/man forms.

So…

                ich = stem (will) + Ø = ich will [I want...]

                du = stem (will) + st = du willst [you want...]

                er/sie/es/man = stem (will) + Ø = er/sie/es/man will [he/she/it/one wants...]