Questions & Answers about Брак был долгим.
Был is the past-tense masculine singular form of быть (to be).
Russian usually does not use to be in the present tense:
- Брак долгий. = The marriage is long. / The marriage is a long one.
(This is grammatical, though not always the most natural sentence in real life.)
But in the past tense, Russian does use forms of быть:
- был — masculine singular
- была — feminine singular
- было — neuter singular
- были — plural
Since брак is masculine singular, the sentence uses был.