"The cow likes the mouse."
Translation:Die Kuh mag die Maus.
If by "per Duolingo", you mean that those two forms are both present in the translation hints for "der", please remember that the hints are not sentence-specific and may contain translations that are not appropriate in the sentence you are currently translating.
der may be:
- masculine nominative
- feminine genitive
- feminine dative
- plural genitive
However, here we have two feminine nouns (Kuh, Maus), one in the nominative case (the subject Kuh) and one in the accusative case (the direct object Maus).
Thus we need the feminine nominative article die before Kuh and the feminine accusative article die before Maus.
der is not appropriate anywhere in this sentence.
Why is it mag and not magt?
For the same historical reasons that we say "he may" and not "he mays".
(Those verbs form their present tense like the past tense of strong -- vowel-changing -- verbs, so it's "I may, you may, he may" just like "I lay, you lay, he lay" or ich mag, du magst, er mag just like ich lag, du lagst, er lag with no endings for the ich or er forms.)
gotcha, that makes sense. I was being led down the idea that it was always ich verb + nothing or 'e', du verb + st, her/sie verb + ht or t. Curiously enough it still accepted my answer even though I typed magt. I just noticed that below it came up as a typo. Probably should be considered a mistake as it misleads people like me who click quickly to the next phrase.
I was being led down the idea that it was always ich verb + nothing or 'e', du verb + st, her/sie verb + ht or t.
ich is always -e (not nothing -- at least in the standard language), du is -st (but -t after a /s/ sound, spelled s ss ß x z, as in du heißt, du isst, du liest, not du heißst, du issst, du liesst), er is -t (not -ht).
sein (to be) is, of course, irregular; but the modal verbs are very common exceptions as well, not having any ending for ich and er / "I" and "he".
In German, the modal verbs are joined by wissen -- ich weiß, er weiß.
Otherwise, they correspond, more or less: er mag - he may; er muss - he must; er kann - he can; er soll - he shall; er darf - [nothing]; er will - he will.
The meanings have shifted sometimes, of course; er mag is usually "he likes" rather than modal "he may = it's possible that he ...", and *he will" in English is usually simple future rather than expressing desire.