Hot answers tagged morphology
Historically the modal verbs derive from the perfect tense which took over a meaning referring to present time. That's why the forms of the modal verbs don't follow the normal pattern. German wissen, not exactly a modal verb, has a similar conjugation in present tense. It is connected with the Latin perfect vidi - I have seen. If you have seen something ...
As rogermue already pointed out, these verbs once were past tense forms: Verbs that had a shift were past tense forms gained a present tense meaning like muss/müssen are called preterite presents or Präteritopräsentien and most of these verbs are the modals that you gave. In former times, there was a difference in singular and plural for strong past tense ...
For a word where both -es and -s are allowed genitive forms, -es is indeed considered more poetic, but could also be interpreted as outdated as stated by Maico Heere. The Duden says that -es is preferred in constellations where the genetive is prefixed (e.g. des Tages Hitze) or within compounds with interfix-s (des Geschäftsfreundes).
Only top voted, non community-wiki answers of a minimum length are eligible