Learn Hardcore French: Ils mangent des oranges fraîches, et ils ne prennent que cela pour le petit-déjeuner. - They eat fresh oranges, and they have only that for breakfast.

Because des is the indefinite plural article used to mean “some oranges” or “oranges in general,” whereas les would refer to specific oranges previously identified or known to the listener.

Why is “only” translated as ne...que here, and why is there no pas?

The construction ne...que is a restrictive negation that means only rather than a full negation like ne...pas. It limits the action or quantity (“they take only that”) instead of negating it entirely, so you don’t need pas.

Why do we use prendre instead of manger for breakfast?

In French, it’s idiomatic to prendre (take) meals: prendre le petit-déjeuner, prendre le dîner, etc. You can use manger, but prendre is the usual verb when naming meals.

Why does the sentence use cela and not ça or il?

Cela is the formal third-person demonstrative pronoun meaning “that (thing).” It refers back to the idea of eating fresh oranges. Ça is more colloquial, and il is only a subject pronoun, so it can’t stand for “that thing.”

Why is fraîches spelled with -es at the end?

Oranges is feminine plural. French adjectives agree in gender and number with their nouns, so frais becomes fraîches (adding -es for feminine plural).

Why is there a hyphen in petit-déjeuner?

Petit-déjeuner is a compound noun in French, and the standard orthography uses a hyphen between petit and déjeuner.

Can you omit the second ils in the sentence?

Yes. You could write:
Ils mangent des oranges fraîches et ne prennent que cela pour le petit-déjeuner.
Repeating ils adds clarity or emphasis, but omitting it in the second clause is grammatically fine.