Duono de la ŝtofo estas kotono.

Duono is a noun meaning half. The ending -o marks it as a noun.

In this sentence, duono de la ŝtofo means half of the cloth/fabric, and that whole phrase is the subject.

By contrast, duon- often appears as a root in compounds, such as duonhoro (half an hour), or in forms like duone (halfway / partly).

This is a very common question.

Use de when you mean a part of a specific whole:

duono de la ŝtofo = half of the cloth

Use da when you mean an amount or quantity of something, usually an unspecified substance:

kilo da sukero = a kilo of sugar

So here, de is right because the sentence is talking about half of a particular piece of cloth/fabric, not just some quantity of cloth in general.

La means the, so la ŝtofo means the cloth / the fabric.

It shows that the speaker has a specific cloth or fabric in mind, not cloth in general. So the sentence is talking about half of the specific fabric already known from the context.

Without la, ŝtofo would sound more general.

Because there is no direct object here.

• Duono de la ŝtofo is the subject.
• estas is a linking verb (is).
• kotono is a predicate noun, not a direct object.

Also, after de, Esperanto does not use the accusative -n.

So everything here stays without -n.

Kotono is a noun meaning cotton.

After estas, Esperanto can use a noun to say what something is made of:

ĝi estas kotono = it is cotton

If you used kotona, that would be an adjective:

kotona ŝtofo = cotton cloth / cotton fabric

So in this sentence, kotono is naming the material itself.

Yes. Estas el kotono means is made of cotton, and it is very clear and natural.

So these are both understandable:

• Duono de la ŝtofo estas kotono.
• Duono de la ŝtofo estas el kotono.

The version with el emphasizes the material a bit more explicitly. The original sentence is shorter and simply identifies that half as cotton.

Ŝtofo usually means cloth, fabric, or textile material.

It is a mass noun, like English cloth or fabric, so singular is normal here. The sentence is not talking about separate countable items, but about material.

Yes.

The full subject is duono de la ŝtofo = half of the cloth.

Inside that phrase:

• duono = half
• de la ŝtofo = of the cloth

Then the rest of the sentence, estas kotono, tells you what that subject is.

Esperanto word order is fairly flexible, but Duono de la ŝtofo estas kotono is the most neutral and straightforward order.

If you changed the order, you might change the emphasis. For example, putting kotono first would make cotton more prominent, but the original sentence is the most natural choice for a simple statement.

Ŝ is pronounced like English sh.

So ŝtofo is pronounced roughly SHTO-fo, with the stress on the second-to-last syllable: ŜTO-fo.

Likewise:

• duono → du-O-no
• kotono → ko-TO-no

Esperanto always stresses the next-to-last syllable.