El sillín es cómodo.

Sillín usually means a saddle or a bike seat. In everyday Latin American Spanish, it often refers to the seat on a bicycle, motorcycle, or something saddle-shaped.

It is more specific than asiento, which just means seat in a general way.

• sillín = saddle / bicycle seat
• asiento = seat

So El sillín es cómodo sounds like The saddle/bike seat is comfortable.

Because sillín is a masculine noun in Spanish.

In Spanish, nouns have grammatical gender, so you have to learn whether a noun takes:

• el for masculine
• la for feminine

Here, the correct form is:

• el sillín

Even though many nouns ending in consonants are masculine, you should still learn the gender with the noun.

Because adjectives in Spanish usually agree with the noun they describe in gender and number.

Since sillín is:

• masculine
• singular

the adjective must also be masculine singular:

• cómodo

If the noun were feminine, you would use:

• cómoda

Examples:

• El sillín es cómodo.
• La silla es cómoda.

Spanish has two verbs that both can mean to be:

• ser → es
• estar → está

Here, es is used because cómodo is describing a more general or inherent quality of the saddle/seat: it is comfortable by design or nature.

• El sillín es cómodo. = The saddle is comfortable.

If you said está cómodo, that would sound unusual here, because estar cómodo is more often used for a person or animal being comfortable in a situation.

Examples:

• Estoy cómodo. = I am comfortable.
• El sillín es cómodo. = The saddle is comfortable.

In Spanish, adjectives often come after the noun, unlike in English.

So instead of:

• comfortable saddle

Spanish commonly says:

• sillín cómodo

In your sentence, the structure is:

• El sillín = the saddle
• es cómodo = is comfortable

This is a very normal word order in Spanish.

The accent marks show which syllable is stressed.

• sillín → stress on the last syllable: si-LLÍN
• cómodo → stress on the first syllable: CÓ-mo-do

These accents are important because they affect pronunciation and sometimes distinguish words.

For a learner, a good rough pronunciation guide is:

• sillín ≈ see-YEEN or see-JEEN, depending on region
• cómodo ≈ KO-mo-do

It depends on the region.

In much of Latin America, ll is pronounced like:

• the y in yes

So sillín may sound roughly like:

• see-yeen

In some places, it can sound more like a soft j sound, and in a few regions even closer to sh.

For most learners of Latin American Spanish, pronouncing it like y in yes is a good default.

Yes, you can, but it is not exactly the same.

• El asiento es cómodo = The seat is comfortable.
• El sillín es cómodo = The saddle/bike seat is comfortable.

Asiento is broader and more general. Sillín is more specific.

So if you are talking about a bicycle seat, sillín is often the better word.

Usually, no.

In a normal sentence like this, Spanish needs the article:

• El sillín es cómodo.

Without it:

• Sillín es cómodo.

that sounds wrong in standard Spanish.

Spanish uses definite articles more often than English does, especially when talking about a specific object or a known thing.

Yes.

To make the sentence plural, both the noun and the adjective change:

• El sillín es cómodo. = The saddle is comfortable.
• Los sillines son cómodos. = The saddles are comfortable.

Notice the changes:

• el → los
• sillín → sillines
• es → son
• cómodo → cómodos

This is another example of agreement in Spanish.

It can describe both.

For objects:

• El sillín es cómodo. = The saddle is comfortable.

For people:

• Estoy cómodo. = I am comfortable.
  If the speaker is female, she would usually say:
• Estoy cómoda.

So cómodo/cómoda changes depending on who or what is being described.

Yes, it is completely natural.

A speaker might say it when talking about:

• a bicycle
• a motorcycle
• a horse saddle
• exercise equipment with a saddle-style seat

It is a simple, standard sentence:

• El sillín es cómodo.

Nothing about it sounds strange or overly formal.