Questions & Answers about Вчера ужин был холодный.
Why do we use был for "was" in this sentence?
Why does the adjective холодный end in -ый?
Adjectives always have to match the noun they describe. Since ужин is a masculine, singular noun in its basic dictionary form (because it is the subject of the sentence), the adjective for "cold" takes the standard masculine ending -ый.
Could I change the word order and say Вчера был холодный ужин?
Yes, but it changes the meaning slightly. Your sentence Вчера ужин был холодный means "Yesterday the dinner was cold," describing the state of a specific meal. Changing the order to Вчера был холодный ужин sounds more like "Yesterday there was a cold dinner." Russian word order is flexible, and putting the adjective at the end emphasizes the description.
Why is there no word for "the" before dinner?
Russian does not use articles like "a" or "the". We just say ужин, and you can tell from the context of the sentence that we are talking about a specific dinner from yesterday, making "the dinner" or just "dinner" the most natural English translation.