Questions & Answers about Ég borða eggið í morgunmat.
Eggið means the egg, while egg by itself just means egg / an egg / eggs (Icelandic has no separate word for a/an).
- egg = base form (nominative/accusative singular, indefinite)
- eggið = egg
- -ið, the definite ending for neuter nouns in the singular accusative (and nominative)
So Ég borða eggið is I eat the egg, talking about a specific egg.
If you wanted to say I eat egg(s) for breakfast in a general way, you would more likely say Ég borða egg í morgunmat (no -ið).