Hi ha una cadira al costat de la finestra.

Hi ha means there is or there are.

It is made of:

• ha = has (from the verb haver)
• hi = a small adverb-like word that is part of this expression

Together, haver-hi is the normal Catalan way to express existence:

• Hi ha una cadira = There is a chair
• Hi ha dues cadires = There are two chairs

So even though ha by itself means has, hi ha functions as a fixed expression meaning there is/are.

In English, we say There is a chair..., which uses is. In Catalan, existence is usually expressed with haver-hi, not with ser or estar.

So:

• Hi ha una cadira... = There is a chair...

Using és or està here would not be the normal way to say that something exists in a place.

A useful contrast:

• Hi ha una cadira a l’habitació = There is a chair in the room
• La cadira és a l’habitació or La cadira està a l’habitació = The chair is in the room

The first introduces the existence of a chair. The second talks about a specific chair already known.

Una cadira means a chair. Catalan usually uses the indefinite article here, just like English.

• una = a / one
• cadira = chair

So:

• Hi ha una cadira... = There is a chair...

You would not normally leave out una in this sentence.

Al costat de means next to, beside, or more literally at the side of.

It is built from:

• a = at / to
• el costat = the side
• de = of

So:

• al costat de la finestra = next to the window

This is a very common expression for location.

Examples:

• al costat de la porta = next to the door
• al costat del llit = next to the bed

Because a + el contracts to al in Catalan.

So:

• a + el costat → al costat

This is similar to how:

• de + el → del

Examples:

• Va al mercat = He/She goes to the market
• El llibre del professor = The teacher’s book

So al costat de is just the normal contracted form.

Because Catalan normally uses the definite article here: la finestra = the window.

So:

• de la finestra = of the window
• al costat de la finestra = next to the window

Just as in English, you usually say next to the window, not next to window.

Also note:

• de + la does not contract
• Only de + el becomes del

So:

• de la finestra = correct
• del llibre = correct for de + el llibre

The sentence follows this structure:

• Hi ha = existential expression, there is
• una cadira = the thing that exists
• al costat de la finestra = location

So literally:

• Hi ha
  • a chair
    • next to the window

This is the normal order in Catalan for this kind of sentence.

Catalan still uses hi ha in the plural.

• Hi ha una cadira = There is a chair
• Hi ha dues cadires = There are two chairs

Unlike English, the verb form does not change here:

• there is
• there are

But in Catalan:

• hi ha works for both singular and plural in modern standard usage

In standard Catalan, hi ha is pronounced approximately like ee AH.

A few helpful points:

• hi sounds like ee
• ha has a clear a sound, like ah
• the h is silent

So the beginning of the sentence sounds roughly like:

• ee AH OO-nə kə-DEE-rə...

Exact pronunciation varies somewhat by dialect, but the main thing is:

• silent h
• hi pronounced like ee

Yes, cadira is feminine.

You can tell because it uses:

• una cadira
• not un cadira

Also:

• la cadira = the chair

So the article shows the gender:

• una / la = feminine
• un / el = masculine

This matters because articles and some adjectives must agree with the noun.

Yes. Prop de la finestra means near the window.

But it is not exactly the same as al costat de la finestra:

• al costat de = right next to / beside
• prop de = near / close to

So:

• Hi ha una cadira al costat de la finestra = the chair is right beside the window
• Hi ha una cadira prop de la finestra = the chair is near the window, but not necessarily immediately beside it

Yes, but it means something slightly different.

• Hi ha una cadira al costat de la finestra introduces the existence of a chair there: There is a chair next to the window
• La cadira és al costat de la finestra refers to a specific chair already known: The chair is next to the window

So both are grammatical, but they are used in different situations:

• Hi ha... = introducing something
• La cadira... = talking about a specific, identified object