"A man and a dog"
Translation:Ein Mann und ein Hund
You use ein before words which are masculine or neuter, and eine before words which are feminine.
The grammatical gender of a word is something that you have to learn together with the word -- you just have to remember that, for example, Hund "dog" is masculine, Katze "cat" is feminine, and Pferd "horse" is neuter.
The grammatical gender of a noun does not necessarily correspond to anything in real life; for example, Mädchen "girl" is grammatically neuter and Löffel "spoon", an inanimate object, is grammatically masculine.
It really is just something you have to memorise.
Because the two nouns are joined by "and", they have to have the same function in the sentence.
They can both be the subject and stand in the nominative case, ein Mann und ein Hund.
Or they can both be the direct object and stand in the accusative case, einen Mann und einen Hund.
But having the two words be in different cases doesn't work. It would be like saying "Him and I went to the library" or "Tom saw him and I yesterday". Either "him and me" or "he and I", but "him and I" is never correct.
I can not get my head around when to use ein, eine or einen. I totally understand die, der, das and even den. (although i do still get den wrong sometimes.) but I just dont understand when to use ein, eine or einen. Is there anybody who could help me with A way to understand which one to use and when please.
I'm guessing that you wrote eine Hund rather than ein Hund.
Duolingo seems to correct from left to right -- once it's "grabbed onto" a word that exists in a correct sentence, it assumes that any different words later on are wrong, i.e. it assumes that eine Hund was wrong for eine Hündin rather than for ein Hund.
You just have to learn and memorise which noun takes which article.
I suggest learning not Hund = dog but instead der Hund = the dog etc. so that you will remember that Hund is a der word (masculine), and similarly die Katze = the cat, das Pferd = the horse etc. so that you know that Katze is feminine, Pferd is neuter, and so on.
Why do you think that?
That would be in the accusative case, which would be used - for example - when those words are the object of a verb. But there is no verb here, or anything else that would require the accusative case.
When quoting words by themselves, we use the nominative case.
Also, Mann and Hund have to be capitalised.