Questions & Answers about Ayer fue un día muy feliz.
Why do we use fue instead of era here?
Both mean "was", but we use the preterite tense (fue) here because of ayer (yesterday). When you sum up a completed event or a specific time period that has a clear beginning and end, you use the preterite. If you were describing ongoing background details without a clear end, you would use the imperfect tense (era).
Since día ends in "-a", shouldn't it be una día?
Día is one of the most common rule-breakers in Spanish! Even though it ends in "-a" (which usually makes a word feminine), día is actually a masculine noun. That is why it takes the masculine word for "a", which is un.
Does feliz change if it describes a feminine word?
No, it stays the same. Adjectives that end in a consonant (like the "-z" in feliz) or the letter "-e" do not change for gender. They look exactly the same whether they describe a masculine noun like día or a feminine noun like noche.