• Igen, ez a szék.

When you use determiners ez/az (this/that), you ALWAYS have to use the definite article, even though it doesn't translate to anything in English.

That means "that window" would therefore be "Az az ablak"

You also have to decline it based on case, but don't worry about that until way later. If you're curious though, if "this window" is a direct (accusative) object, you would have "ezt az ablakot", and "on that table would be "azon az asztalon."

Like in a conversation:

• Do you want to take one of the chairs?

• Yes, this chair.

• El akarod vinni az egyik széket?

• Igen, ezt a széket.

Igen ez a szék

That's also correct :)

I think that actually sounds way more natural. I don't think anyone in reality would say "Igen, ez szék" instead of "Igen, ez egy szék"

I would use "Ez a szék". But some native Hungarian must confirm this first :)

I want to know this as well

In constructions with "A is B" where B is a noun or adjective, the ungarian translation is "A B." (This is only true in third person singular, present tense, indicative mood.)