"This is the ocean."
Translation:Αυτός είναι ο ωκεανός.
Hello! Firstly, it is αυτό or αυτός (or αυτή), spelled with a υ=Υ not a ν=Ν. :) 'Ocean' is a masculine word in Greek, so you have to use αυτός. Αυτό is used for neutral words and αυτή for feminine ones. So, you need to learn every word with its grammatical gender and then have the articles/pronouns/adjectives agree with that!
Verbs end either in -ω (active voice) or in -μαι (passive voice - in Greek this means a different thing than it does in English). There are several verb types with distinct conjugation patterns within each voice, but they sort of follow the same overall idea.
You can review the Tips and notes at https://www.duolingo.com/skill/el/Verbs%3A-Present/tips-and-notes to see how the two conjugation tables differ in general. The best thing to do is learn the endings and figure out how to add them to each verb stem. This might not work every time, but you'll learn to recognise the patterns more easily. :) For the active voice for example:
|ending||example 1||example 2||example 3|
Thank you for your excellent answer. I see there are some elisions going on, but apart from that, there is nothing surprising. I recognise the -ουν in 3p pl. from Latin -(u)nt, the -εις from Latin 2p sg -(ā/ē/i/ī)s and of course the 1p sg -ω exactly matches the Latin -ō. I still am a bit confused by -ναι/-μαι, but I’m sure that confusion will be cleared once I’ve read the link you provided.