Questions & Answers about La zipo estas en la ŝtofo.
La is the Esperanto definite article, equivalent to the in English. It does not change form.
So:
- la zipo = the zipper
- la ŝtofo = the fabric / the cloth
It is used here because the sentence is talking about specific things, not just any zipper or any fabric.
Esperanto normally has no indefinite article. English distinguishes between a zipper and the zipper, but Esperanto usually only marks the definite one with la.
So:
- zipo = a zipper or just zipper
- la zipo = the zipper
That means Esperanto learners often need to get used to the idea that no article can already mean something like a/an.