Meine or Mein? Ein or Eine?
How do i know if it is Meine or Mein, eine or ein. And how do i know which endings there has to be on the word.. Fx Fleugzeug. Duolingo says it's Flugzeuge, but why not Flugzeugen?
ein Mann | der Mann | mein Mann (masculine)
eine Frau | die Frau | meine Frau (feminine)
ein Kind | das Kind | mein Kind (neutral)
For plurals you can look at this page: http://germanforenglishspeakers.com/nouns/gender-and-plurals/
German plurals are a bit random, and there are a lot of different endings any given word may take to make it plural. But each particular word is only changed one way. "Flugzeuge" is simply just the plural of "Flugzeug".
As far as mein and meine goes, they change depending on the gender of the noun that they precede. For example, "Katze" is feminine, so one would say "meine Katze". But "Hund" is masculine so you'd say "mein Hund". The only way to know the ending of these words, or any other adjectives, is to know the gender of the noun they describe.