Scaunul de lângă ușă este liber.

Because Romanian usually puts the definite article at the end of the noun, not before it like English does.

• scaun = chair
• scaunul = the chair

So Scaunul means the chair.

This is very common in Romanian:

• băiat = boy
• băiatul = the boy
• masă = table
• masa = the table

Here, de lângă means something like by, next to, or near.

So:

• de lângă ușă = by the door / next to the door

In this sentence, it identifies which chair we are talking about: the one near the door.

A helpful way to think about it is:

• scaunul de lângă ușă = the chair that is near the door

Yes, lângă by itself means near / next to. But after a noun, Romanian often uses de lângă to attach that location phrase to the noun more naturally.

So:

• scaunul de lângă ușă = the chair by the door

This is a very normal pattern in Romanian. It works a bit like a compact way of saying:

• scaunul care este lângă ușă = the chair that is near the door

So the de is part of a common structure used to describe or identify a noun.

Because after a preposition like lângă, Romanian often uses the noun without the definite article when referring to a location in a general, natural way.

So:

• lângă ușă = by the door

Even though English says the door, Romanian often does not need the definite form here.

That said, Romanian can use a definite noun after a preposition when the door is being specified more clearly:

• lângă ușa de la intrare = next to the entrance door

So ușă in your sentence is completely natural.

Este liber literally means is free, but in this context it means is available or is unoccupied.

So the sentence means that the chair is not being used.

For objects like seats, chairs, tables, or rooms, liber often means:

• free
• empty
• available

So Scaunul de lângă ușă este liber means the chair by the door is available to sit on.

Because liber has to agree with scaunul, and scaun is a masculine singular noun.

So:

• scaunul = masculine singular
• therefore liber = masculine singular form

Compare:

• scaunul este liber = the chair is free
• masa este liberă = the table is free

So the ending changes depending on the gender and number of the noun.

In standard Romanian, you normally do need the verb to be here.

• este = is

So:

• Scaunul ... este liber = The chair ... is free

In everyday speech, Romanians very often shorten este to e:

• Scaunul de lângă ușă e liber.

That is very common and natural in conversation. But este is the fuller, more formal form.

A rough English-friendly pronunciation is:

SKA-oo-nul deh LUN-guh OO-shuh ES-teh LEE-ber

A few important sounds:

• ș sounds like sh
  • ușă ≈ oo-shuh
• ă is a short neutral vowel, like the a in sofa
• â has a special Romanian sound with no exact English equivalent
  • in lângă, it is the vowel in the first syllable

Also note that scaun has two vowel sounds in a row:

• scaun ≈ ska-oon

It breaks down like this:

• Scaunul = the chair
• de lângă ușă = by the door
• este = is
• liber = free / available

So the structure is:

• subject: Scaunul de lângă ușă
• verb: este
• predicate adjective: liber

In other words:

• The chair by the door
  • is
    • free

That sounds less natural in standard Romanian.

The usual form is:

• Scaunul de lângă ușă este liber.

That de helps connect the location phrase to the noun scaunul.

If you leave it out, Romanian speakers may still understand you, but it does not sound as correct or as natural. So for this kind of phrase, it is best to learn:

• noun + de lângă + place

Example:

• cartea de lângă lampă = the book by the lamp
• scaunul de lângă ușă = the chair by the door