Questions & Answers about Byla to dlouhá doba.
This is a very common question.
Even though to is a neuter word, in this kind of Czech sentence the verb often agrees with the noun that identifies what to is — here, doba.
So:
Byla to chyba. = It was a mistake.
(chyba is feminine, so byla)Byl to problém. = It was a problem.
(problém is masculine, so byl)Bylo to město. = It was a town/city.
(město is neuter, so bylo)
So in Byla to dlouhá doba, the feminine noun doba controls the form byla.