Learn Hardcore Russian: Потолок в комнате был белым.

был is the masculine-singular past-tense form of быть (“to be”). In Russian, when you describe something in the past, you generally need a form of быть to link the subject and its characteristic (in this case, the color of the ceiling).

Why is the adjective белым in the instrumental case instead of the nominative белый?

In Russian, predicate adjectives after a past-tense form of быть (and in the simple future) normally take the instrumental case. That’s why белым (instr. masc. sg.) is used instead of the nominative белый.

What case is комнате in, and how do you form it?

комнате is in the prepositional case (used for location “in/on/at”). To form the prepositional of a feminine noun ending in -а (like комната), you drop -а and add -е: комната → комнате.

Why do we need the preposition в here? Could we just say комнате?

You need в to express “inside” or “within.” Russian prepositions like в (in) govern the prepositional case when indicating location. Without в, комнате would just look like a standalone case form without a clear meaning.

What gender and number is потолок, and how do you recognize it?

потолок is masculine singular. You can tell because it ends in a consonant (typical pattern for most masculine nouns), and it agrees with the verb form был (masc. sg.) and the instrumental adjective белым (masc. sg.).

How would you say “The ceiling in the room is white” in the present tense?

In the present tense, Russian often drops the verb быть altogether. So you’d say:
Потолок в комнате белый.

Can I change the word order for emphasis?

Yes. Russian is fairly flexible. For example:
• В комнате потолок был белым. (Emphasizes в комнате)
• Белым был потолок в комнате. (Emphasizes белым)
All these versions keep the same basic meaning but shift the focus.