"My mother swims more than my father."
Translation:Meine Mutter schwimmt mehr als mein Vater.
Is it not all nominative? (Not being sarcastic, my grammar education was pathetic) To my understanding, the direct object gets the accusative. The father isn't being "verbed" in this sentence, therefor not the direct object, and the mother isn't doing something to or for the father, so he isn't the indirect object, therefor not dative. And it isn't genitive. Default tells me it's nominative. (?)
This is difficult for Anglophones because in english both "than I/he/she/we/they" and "than me/him/her/us/them" are accepted (subjective and objective cases). However, only the former is correct, and it seems to be the same in German. Remeber that in "my mother swims more than my father (swims)", "swims" is implied, and so "my father" is a subject, not an object.
"The comparison of adjectives and adverbs in German: No matter how long the adjective or adverb, German always adds -er ("schöner", "interessanter"). Never use mehr for this purpose. Adjective endings follow the -er. Of course, adverbs and predicate adjectives take no endings."
That would be a translation for "My mother swims more than my papa".
A quite an unlikely sentence in either language, mixing more formal Mutter/mother with more informal Papa/papa. Unless, perhaps, the speaker wants to emphasize the different levels of respect and affection for each parent.