How to know when to use the and when not to use
Could "elmaları" translate as "his/her/its apples?" I.e. could it be "The elephants eat his/her/its apples?"
Not quite. Since his/her/its apples would always be specific (you are referring specific apples), you would need both the accusative and the 3rd person possessive suffix. That would mean the sentence would be "Filler elamlarını yer."
How do we know when to use 'ler' or 'lar' after the subject? E.g. Filler, Atlar
From Turkish lesson "Plurals": " Basically if the final vowel is front (i, e, ü, ö) use -ler. If it is back (a, ı, o, u), use the suffix -lar."
Turkish is really tough