El pomo de la puerta está suelto.

Spanish usually links two nouns with de where English often puts one noun in front of another.

So:

• the door knob → el pomo de la puerta
• literally: the knob of the door

This is very common in Spanish:

• the car door → la puerta del coche
• the kitchen table → la mesa de la cocina

So de la puerta is the normal Spanish way to express door as a modifier.

In Spain, pomo usually means a knob, especially a round door knob.

Related words you may also hear:

• manilla / manivela / manija depending on region and object
• tirador = a pull handle
• picaporte = latch/door handle area in some contexts

For this sentence, pomo is best understood as doorknob.

Because estar is used for a state or condition, while ser is used more for identity, classification, or inherent characteristics.

Here, the knob is in a temporary or current condition: it is loose.

So:

• El pomo está suelto. = The knob is loose.

Using es suelto would sound wrong here.

A useful comparison:

• La puerta está abierta. = The door is open.
• La puerta es de madera. = The door is made of wood.

Because suelto agrees with pomo, and pomo is a masculine singular noun.

Agreement in Spanish means the adjective changes to match the noun:

• el pomo → suelto
• la puerta → suelta
• los pomos → sueltos
• las puertas → sueltas

Even though puerta is feminine, the adjective is describing pomo, not puerta.

Here, suelto means loose, as in not firmly attached.

It can have other meanings in other contexts, for example:

• pelo suelto = loose hair / hair worn down
• andar suelto = to be roaming around / loose
• monedas sueltas = loose change

But in El pomo de la puerta está suelto, the meaning is clearly loose in a physical sense.

Sometimes yes, but suelto is more natural here for something that is physically unattached or wobbling.

Compare:

• está suelto = it is loose / not secure
• está flojo = it is loose/slack, often with the idea of not tight enough

For a doorknob, suelto is a very good choice.
For a screw or a belt, flojo is also very common:

• El tornillo está flojo. = The screw is loose.

Spanish normally uses articles more often than English.

In this sentence:

• el pomo = the knob
• de la puerta = of the door

So both nouns take their own article. That is completely normal in Spanish.

You would not usually say just pomo de puerta in a normal sentence like this.

Yes, but the original order is the most neutral and natural:

• El pomo de la puerta está suelto.

You might also hear:

• Está suelto el pomo de la puerta.

This can sound a bit more emphatic or descriptive, for example when noticing the problem.

Still, for most learners, the standard order is the best one to use first.

You can simply raise your voice in speech, or use question marks in writing:

• ¿El pomo de la puerta está suelto? = Is the door knob loose?

Spanish does not usually need do/does/is/are support like English does. The verb stays the same.

A simple pronunciation guide is:

el PO-mo de la PWER-ta es-TA SWEL-to

A few points:

• pomo → stress on PO
• puerta → stress on PWER
• está → stress on the last syllable because of the accent mark
• suelto → roughly SWEL-to

In standard Spain Spanish:

• c before e/i sounds like th in think, but there is no c here
• z is not present here either
• u in puerta is pronounced

Yes, if the context is already clear.

• El pomo está suelto. = The knob is loose.

Adding de la puerta makes it specific:

• maybe not a drawer knob
• not some other knob
• specifically the door knob

So both are correct; the longer version is just more precise.