"The fox is a mammal"
Translation:Η αλεπού είναι θηλαστικό
Yes, initially I struggled with this a bit. There are quite a few rules, but they seem to just boil down to the advice that Dimitra gave - you really only need to use indefinite articles if there could be confusion about quantity. So if you're talking about abstract or uncountable things, you never need to use them. In the above sentence, the fox is never going to be two mammals, so there's no need to use it. Likewise for something like 'αυτοί δεν είναι ζευγάρι' - they are not a couple - it's never going to be 'they are not two couples', so you don't need the indefinite article.
For something like... I want an apple, you'd have 'θέλω ένα μήλο', and use the indefinite article, as you could conceivably want two apples if you were hungry, or want to make clear to the grocer that you only want to buy one apple.
I think that's right, anyway!