Byla to dlouhá doba.

Byla is the past tense, feminine singular form of the verb být (to be).

A very quick pattern is:

• byl = masculine singular
• byla = feminine singular
• bylo = neuter singular
• byli / byly = plural forms

In this sentence, byla is used because the key noun is doba, and doba is feminine singular.

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.

Here to works like it / that / this in an identifying sentence.

So the structure is roughly:

• Byla to ... = It was ... / That was ...

It is very common in Czech to use to in this pattern when you are identifying or describing something.

Without to, the sentence would usually sound incomplete or unnatural in normal speech.

Because dlouhá is an adjective modifying doba, and Czech adjectives must agree with the noun they describe in:

• gender
• number
• case

Since doba is:

• feminine
• singular
• nominative

the adjective also has the feminine singular nominative form:

• dlouhý = masculine
• dlouhá = feminine
• dlouhé = neuter

So:

• dlouhá doba = a long period/time

Doba is in the nominative singular.

That is because it is the noun naming what the thing was: a long period.

In this sentence type with to + být, Czech normally uses the nominative:

• Byla to dlouhá doba.
• Byl to těžký den.
• Bylo to zajímavé období.

So both dlouhá and doba are nominative singular feminine.

Not exactly.

Both relate to time, but they are used differently:

• čas = time in a general sense, clock time, free time
• doba = a period, span, stretch, or era

In this sentence, doba is natural because the idea is a long period of time.

Compare:

• Nemám čas. = I don’t have time.
• Byla to dlouhá doba. = It was a long period / It was a long time.

So doba here is better than čas.

Yes. To byla dlouhá doba is also correct.

Czech word order is more flexible than English, and changing the order often changes emphasis more than basic meaning.

Very roughly:

• Byla to dlouhá doba. = a bit more narrative or descriptive
• To byla dlouhá doba. = often gives more emphasis to that/it

In many contexts, both would be understood the same way.

Usually no, not in normal everyday Czech.

Byla dlouhá doba sounds incomplete or unnatural in most ordinary contexts.

The to is an important part of this identifying pattern:

• Byla to dlouhá doba. = natural
• Byla dlouhá doba. = usually not natural

So as a learner, it is safest to keep to.

A simple pronunciation guide is:

BY-la to DLOU-ha DO-ba

A few useful points:

• Czech stress is usually on the first syllable of each word.
• ou in dlouhá is a real diphthong, roughly like oh-oo blended together.
• á is a long vowel, so the last syllable of dlouhá is held a bit longer.

Very approximate English-style guide:

BIH-lah toh DLOH-haa DOH-bah

Not exactly.

Byla to dlouhá doba literally means It was a long time / It was a long period.

That can suggest duration, not necessarily distance in the past.

If you want That was a long time ago, Czech would more naturally say something like:

• To bylo dávno.
• Bylo to už dávno.

So:

• Byla to dlouhá doba = the period itself was long
• Bylo to dávno = it happened long ago

It is not completely fixed, but it is a very common natural combination.

Czech often uses dlouhá doba for a long period / a long time.

You will also see related expressions like:

• krátká doba = a short period
• po dlouhé době = after a long time
• dlouhou dobu = for a long time

So it is a useful phrase pattern to remember, not just a one-off sentence.