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.

Because after был in this kind of sentence, Russian often uses the instrumental case for the predicate adjective.

So:

• dictionary form: долгий = long
• instrumental masculine singular: долгим

That is why we get:

• Брак был долгим.

This pattern is very common with nouns and adjectives after past/future forms of быть:

• Он был врачом. = He was a doctor.
• День был трудным. = The day was difficult.

You may sometimes see nominative instead, but instrumental is the standard and very common choice here.

Russian adjectives must agree with the noun they describe in gender, number, and case.

Here:

• брак = masculine, singular
• the adjective must therefore also be masculine, singular
• because of the sentence structure, it appears in the instrumental case

So:

• masculine singular nominative: долгий
• masculine singular instrumental: долгим

Everything matches брак.

Брак is in the nominative case.

It is the subject of the sentence:

• Брак = the marriage
• был долгим = was long / lasted a long time

So the structure is:

• subject in nominative: брак
• past tense of to be: был
• predicate adjective in instrumental: долгим

It can mean two different things:

1. marriage
2. defect / faulty goods / manufacturing flaw

So брак is one of those Russian words whose meaning depends on context.

For example:

• счастливый брак = a happy marriage
• товар с браком = defective goods / goods with defects

In Брак был долгим, the context clearly points to marriage, not defect.

Yes, it is grammatical and natural.

Брак был долгим means something like:

• The marriage was long.
• The marriage lasted a long time.

Another very natural Russian way to express a similar idea is:

• Брак длился долго. = The marriage lasted a long time.

The version with был долгим focuses on the marriage as having the quality of being long, while длился долго focuses more directly on duration.

Yes. Russian word order is flexible, though the emphasis changes.

Neutral order:

• Брак был долгим.

If you move words around, you may highlight a different part:

• Долгим был брак. — more literary or emphatic
• Был долгим брак. — possible, but less neutral

For a learner, Брак был долгим is the safest and most neutral order.

Normally, no.

In the present tense, Russian usually omits есть (is/am/are) in ordinary statements:

• Брак долгий. = The marriage is long.

But in the past tense, you need был / была / было / были:

• Брак был долгим.

So English says is/was, but Russian usually has:

• present: no verb
• past: a form of быть

Russian has both full and short adjective forms, but not every adjective is equally common in short form in every context.

Here, the full adjective in the instrumental case is the normal choice:

• долгим

A short-form version like долог exists, but it sounds more literary or stylistically marked:

• Брак был долог.

That is not wrong, but Брак был долгим is much more straightforward for everyday standard Russian.

A simple pronunciation guide is:

• Брак → brak
• был → roughly byl
• долгим → DOL-geem

A more careful approximation:

• Брак был долгим → brak byl DOL-gim

Stress falls on:

• брак — one syllable
• был — one syllable
• до́лгим — stress on the first syllable

So the main stress in the last word is:

• до́лгим

Usually it refers to duration: the marriage lasted a long time.

But, as in English, describing something as long can sometimes carry a slight implication about how it felt or how the speaker views it. Context decides that.

Still, the basic meaning is simply about length in time:

• долгий брак = a long marriage
• Брак был долгим = The marriage was long / lasted a long time