Breakdown of Мой дедушка был архитектором.
мой
my / mine
дедушка
grandfather
быть
to be
архитектор
architect
Questions & Answers about Мой дедушка был архитектором.
Why does the word for architect end in -ом here?
This sentence uses the Instrumental case, which is the standard rule when stating someone's profession in the past using был (was). Because архитектор is a masculine noun ending in a consonant, we add -ом to make it архитектором.
Why do we use the masculine words Мой and был if дедушка ends in -а?
While most nouns ending in -а are feminine, дедушка (grandfather) refers to a male person. Nouns that name male family members or people are always masculine, so all describing words and past-tense verbs must match them in the masculine form!
Can I just say Мой дедушка был архитектор using the regular dictionary form?
You might occasionally hear native speakers do this in very casual conversation, but using the Instrumental case (архитектором) is the standard and grammatically correct rule for professions with быть (to be) or стать (to become). It is highly recommended to practice it this way.
If I want to say he IS an architect right now, do I still use the Instrumental case?
No, in the present tense, you drop the verb to be entirely and just use the Nominative (dictionary) case: Мой дедушка — архитектор. You only need the Instrumental case for professions in the past (был) or future (будет).