La roverso estas el kotono.

La means the. It shows that the speaker is talking about a specific roverso, not just any one.

Esperanto has only one definite article, la, and it does not change for gender or number.

Estas is the present-tense form of esti, meaning to be.

A useful point for English speakers: Esperanto verbs do not change according to the subject. So you say:

• mi estas = I am
• vi estas = you are
• li estas = he is
• ili estas = they are

So estas always stays the same in the present tense.

Because -o is the normal noun ending in Esperanto.

So:

• roverso is a noun
• kotono is also a noun

This is one of the most regular features of Esperanto: the ending tells you the word class.

Here el means out of or made of.

After esti, el + material is a very common way to say what something is made from. So:

• estas el kotono = is made of cotton

You will see the same pattern in sentences like:

• La tablo estas el ligno = The table is made of wood
• La ringo estas el oro = The ring is made of gold

For physical material, Esperanto normally prefers el.

De has many other meanings, such as of, from, by, or showing a relationship. But when you want to say that something is composed of a material, el is the usual and clearest choice.

So el kotono is the natural way to say made of cotton.

Yes, you could.

Kotona is the adjective cotton or cotton-made. So La roverso estas kotona would also make sense.

The difference is mainly one of emphasis:

• estas el kotono focuses on the material: it is made of cotton
• estas kotona describes it with an adjective: it is cotton / cotton-type

In many everyday situations, both are acceptable, but el kotono is especially clear when talking about what something is made from.

Because kotono here means cotton as a material in general, not a specific amount or batch of cotton.

This works much like English. We usually say made of cotton, not made of the cotton, unless we mean some particular cotton.

If you did mean specific cotton, then la could appear, for example:

• el la kotono, kiun ni aĉetis = from the cotton that we bought

Because there is no direct object here.

• La roverso is the subject
• estas is the verb
• el kotono is a prepositional phrase

In Esperanto, the accusative -n is usually used for direct objects, but this sentence does not have one. Also, after a preposition like el, the noun normally does not take -n.

Yes. This is the most neutral and ordinary word order:

subject + verb + complement

So:

• La roverso = subject
• estas = verb
• el kotono = complement

Esperanto word order is somewhat flexible, but this version is the most straightforward and natural for a learner.