El pedal está bajo el sillín.

Because Spanish normally uses estar for the location of people and things.

• El pedal está bajo el sillín = the pedal is under the saddle.
• Using es here would sound wrong for simple physical location.

A very useful rule is:

• ser = identity, classification, origin, etc.
• estar = location, condition, state

So when you say where something is, estar is usually the verb you want.

Here, bajo means under or below.

In this sentence, it works as a preposition:

• bajo el sillín = under the saddle

So the structure is:

• [thing] + está + bajo + [place/object]

Example:

• El gato está bajo la mesa = The cat is under the table.

Yes. El pedal está debajo del sillín is also correct, and many learners hear debajo de very often in everyday Spanish.

Compare:

• bajo el sillín = under the saddle
• debajo del sillín = underneath the saddle

They are very close in meaning here.

One useful grammar point:

• debajo de + el becomes debajo del
• So not debajo de el sillín, but debajo del sillín

Because pedal and sillín are both masculine nouns in Spanish.

That is why you say:

• el pedal
• el sillín

For nouns ending in a consonant, you often just have to learn the gender with the word. There is not always a simple ending-based rule.

A good habit is to memorize nouns with their article:

• el pedal
• el sillín

Sillín usually means a saddle or seat, especially on a bicycle.

In this sentence, it most likely refers to a bike saddle.

It is more specific than asiento, which is a more general word for seat.

So:

• sillín = saddle/bike seat
• asiento = seat in a general sense

The accent mark in está shows the stress and also helps distinguish it from other words.

• está = is
• esta = this (feminine)

So the accent is important both for pronunciation and meaning.

Without the accent, it becomes a different word.

Because the stress falls on the last syllable: si-LLÍN.

Spanish spelling rules say that words ending in n, s, or a vowel are normally stressed on the next-to-last syllable unless there is a written accent.

Since sillín ends in n but is stressed on the last syllable, it needs the accent mark.

Usually something close to see-YEEN in much of Latin America.

A few notes:

• The stress is on the last syllable: si-llín
• In many places, ll sounds like the English y in yes
• In some regions, especially around Argentina and Uruguay, ll may sound more like sh or zh

So the exact sound can vary by region, but the stressed final syllable stays the same.

No. That would sound unnatural and incorrect in standard Spanish.

Spanish usually needs the article with nouns like these:

• El pedal está bajo el sillín

Both pedal and sillín are specific, countable nouns here, so the definite article is expected.

The normal, neutral order is:

• El pedal está bajo el sillín

But Spanish can change the order for emphasis or style. For example:

• Bajo el sillín está el pedal

That version is grammatical, but it sounds more marked or literary. For everyday use, the original order is the safest choice.

No. They are related, but they are not used the same way.

• bajo = under / below
• abajo = down / below / downstairs

In this sentence, you need bajo because it connects the pedal to the saddle:

• El pedal está bajo el sillín

You could say:

• El pedal está abajo

But that means something more like The pedal is down there/down below, not specifically under the saddle.

Yes. This pattern is very common:

• [object] + está + [place/location phrase]

Examples:

• La mochila está sobre la silla = The backpack is on the chair.
• La llave está en la mesa = The key is on the table.
• El pedal está bajo el sillín = The pedal is under the saddle.

So this sentence is a very useful model for talking about where things are.