But not all the "sentences" used in the introductory Greek lessons are grammatical. They include things like "β βουβάλι", "ω ωκεανός", and "ψ ψυχή." (I think that this is what the original poster meant by "the other ' ' sentences.") It doesn't seem to me that "eta Elena" is any less grammatical than these.
If you are on the web go to the Tips & notes (look for the lightbulb at the start of each lesson.) Or go to our forums https://forum.duolingo.com/comment/22040507
They are used in different classifications of nouns there are three kinds. They are called masculine ο, feminine η, το neuter. Don't be fooled by the masculine etc it doesn't mean they are used exclusively with male, female or neuter it's just a way of grouping. So, the girl is neuter but the chair is feminine. It'll be taught in an upcoming unit. Remember to always learn nouns with their gender "ο", "η" or "το". It will make learning easier.
These links will not only show you how to get the Greek keyboard but also how to find the Greek letters on it, how to add accents etc.
It will also help you learn the alphabet and where to find other useful links.
And here is another to help you navigate Duolingo FAQ - General Questions, Bugs & Reports https://www.duolingo.com/comment/23799672
Some simple rules to get you started:
Always read the comments before posting.
Read the Tips & notes right below the list of lessons on your Homepage
Read the drop down hints. Pass your cursor over a word and a list of words will appear.
If you have questions just ask.