Learn Croatian: Mislimo da je nova kuća prostrana. - We think that the new house is spacious.

In Croatian, je is an enclitic (a weak word that cannot stand alone) and usually needs to take the second position in its clause. The conjunction da acts as the first position of the subordinate clause, so je tucks in right behind it.

Both "nova" and "prostrana" end in "-a". Are they in the same case?

Yes, they are both in the nominative case. nova modifies the subject kuća directly, while prostrana is the predicate adjective describing the house after the verb je. Predicate adjectives must match the subject in gender, number, and case, so both take the feminine singular nominative ending -a.

How would this sentence change to fit this lesson's focus on "sve" (everything), such as "We think that everything is spacious"?

To say "everything is spacious," you replace nova kuća with the universal pronoun sve. Because sve acts as a neuter singular pronoun, the predicate adjective must also take the neuter singular ending: Mislimo da je sve prostrano.

Why isn't the pronoun "mi" (we) included before "mislimo"?

Because the verb ending -mo in mislimo already tells us the subject is "we." In Croatian, you generally drop the subject pronoun unless you want to add special emphasis or contrast.