Good sentence! It is grammatically correct. Add a period at the end to finish the sentence: Koalas eat nothing but eucalyptus leaves.
Here is what the parts mean:
- Koalas — the animals (that is the subject).
- eat — the action they do (the verb).
- nothing but eucalyptus leaves — this means "only eucalyptus leaves" (the object).
Another way to say the same thing is: Koalas eat only eucalyptus leaves.
Nice job! The sentence is correct; just remember the period at the end.