"The cupboard is expensive."

Translation: Der Schrank ist teuer.

[Thorondor95]

So Schrank can mean any storage space, like a closet or a cabinet?

[mizinamo]

Pretty much.

[alan501444]

Why not teurer?

[mizinamo]

Because that would mean "more expensive".

Remember that predicative adjectives (after "to be", etc.) don't have endings for gender, number, and case, so der Schrank ist teurer would have the -er interpreted not as the "masculine nominative singular" ending but as the "comparative" ending.

[MalvinJusu]

Do you have any link about that?

[mizinamo]

I don't, sorry. https://en.wikipedia.org/wiki/German_adjectives may be a start, though.

[khalilelbo1]

Why is it der not das

[mizinamo]

Because Schrank is a masculine noun and so it needs the masculine article der, not the neuter article das.

[pier4r]

I did a mistake, and this was the suggestion "Der Schrank ist teür."

I think here the teuer gets automatically replaced by a (not wanted) umlaut.

[mizinamo]

I think here the teuer gets automatically replaced by a (not wanted) umlaut.

Yes; this is a Duolingo mistake.

teuer is the only correct spelling.