Questions & Answers about Мой день был радостным.
Why is it Мой день, not Моё день?
In Russian, adjectives and pronouns agree with the noun in gender, number, and case. The noun день (“day”) is masculine, so the possessive pronoun must be masculine nominative: мой, not neuter моё.
Why is день in the nominative case and not another case?
Because день is the grammatical subject of the sentence (“my day”). Subjects in Russian appear in the nominative case.
What does был mean and why is it used here?
был is the past-tense form of the verb быть (“to be”). In the past (and future) tense, Russian requires an explicit copula, so “was” must be был for masculine singular.
Why is радостным in the instrumental case instead of the nominative радостный?
When you use a predicate adjective with быть in the past or future, Russian puts the adjective in the instrumental case. The instrumental here marks the state or quality that the subject “was.”
How do you form the instrumental singular masculine of adjectives like радостный?
For hard-stem adjectives ending in -ый, replace -ый with -ым:
радостный → радостным.(Soft-stem adjectives ending in take : синий → синим.)