Learn Croatian: Portret u dvorcu bijaše lijep. - The portrait in the castle was beautiful.

It is the imperfect tense (imperfekt) of the verb biti (to be), specifically the 3rd person singular. The imperfect is an older tense used for past states or ongoing past actions. While you would use the standard perfect tense (je bio) in everyday speech, you will frequently encounter bijaše in literature, historical texts, or poetry to set a scene.

Will I always see bijaše for the imperfect of "to be", or are there other forms?

The verb biti is quite unique because it actually has two sets of imperfect forms. Besides bijaše, you will also frequently see bješe (or sometimes just the short form bje) in written texts. They all mean the exact same thing.

Why does the word for castle become dvorcu here instead of dvoracu?

The dictionary form (nominative) is dvorac. When we add the locative case ending -u after the preposition u (in), the "a" in the final syllable drops out. This is a common noun rule called the "fleeting a" (nepostojano a), which helps make the word smoother to pronounce when suffixes are added.

Why do we use the short adjective form lijep instead of the long form lijepi?

Lijep is the indefinite (short) form of the adjective. In Croatian, when an adjective is used after a linking verb like "to be" to describe the subject—functioning as a predicate adjective—it must be in the indefinite form if the word has one.