Kiedyś mój brat był bardzo cierpliwy, ale teraz jest nerwowy.

Here, kiedyś means once / in the past / back then.

A very useful thing to know is that kiedyś can mean two different things depending on context:

• in the past, once, back then
• sometime, someday

In this sentence, it clearly means in the past, because it is followed by a past-tense verb: był.

So here:

• Kiedyś mój brat był bardzo cierpliwy = My brother used to be very patient / My brother was once very patient

Because mój has to agree with brat in gender, number, and case.

• brat is masculine singular
• so the possessive adjective must also be masculine singular: mój

Compare:

• mój brat = my brother
• moja siostra = my sister
• moje dziecko = my child

So moja brat is incorrect because moja is the feminine form.

Because the sentence contrasts the past with the present.

• był = was
• jest = is

More specifically:

• był is the past tense, masculine singular form of być (to be)
• jest is the present tense form of być

Since brat is masculine, the past tense has to be masculine too:

• on był = he was
• ona była = she was
• ono było = it was

So:

• mój brat był = my brother was
• teraz jest = now he is

Because they are adjectives describing brat, and they must agree with it.

Brat is:

• masculine
• singular
• nominative

So the adjectives also appear in the masculine singular nominative form:

• cierpliwy = patient
• nerwowy = nervous / edgy / irritable

That is why you get:

• brat był cierpliwy
• brat jest nerwowy

This is normal with adjectives used after być.

Because with an adjective after być, Polish normally uses the nominative form, not the instrumental.

So:

• On jest cierpliwy. = He is patient.
• On był nerwowy. = He was nervous.

That is why the sentence has:

• cierpliwy
• nerwowy

and not:

• cierpliwym
• nerwowym

A learner may get confused because with nouns after być, Polish often uses the instrumental:

• On jest lekarzem. = He is a doctor.

But with adjectives, the nominative form is the normal one:

• On jest miły. = He is nice.

Not always exactly.

Nerwowy can mean:

• nervous
• tense
• high-strung
• edgy
• sometimes even irritable

So in this sentence, depending on context, it could suggest that the brother is:

• anxious and tense, or
• easily irritated and no longer calm

That is slightly broader than English nervous, which often focuses more on anxiety.

So it is a good translation, but it is worth remembering that nerwowy can also imply a person who is jumpy, tense, or bad-tempered.

Because Polish often omits subject pronouns when they are already clear from the context.

In English, you must say:

• he is

But in Polish, the verb often gives enough information, and the subject has already been mentioned:

• mój brat był bardzo cierpliwy, ale teraz jest nerwowy

The subject mój brat still applies to the second part, so there is no need to repeat on.

You could say:

• ..., ale teraz on jest nerwowy

but that would usually add emphasis, as if you were stressing he in particular.

Ale means but and gives a clear sense of contrast.

The sentence contrasts two states:

• before: bardzo cierpliwy
• now: nerwowy

So ale is very natural.

Could you use a? Sometimes yes, because a can also connect contrasting ideas in Polish. But the feeling is a bit different:

• ale = stronger but
• a = often softer contrast, something like and/but/whereas

So:

• Kiedyś mój brat był bardzo cierpliwy, ale teraz jest nerwowy.

sounds like a strong contrast and is the most natural choice here.

No, Polish word order is fairly flexible, but this version is the most neutral and natural.

This sentence has a very standard structure:

• Kiedyś = time expression
• mój brat = subject
• był bardzo cierpliwy = predicate
• ale teraz = contrast + time expression
• jest nerwowy = predicate

Because Polish uses endings to show grammatical relationships, you can often move words around for emphasis. For example, you might hear different orders in speech or writing, but they would change the focus.

The given sentence is a good neutral pattern for learners:

• Kiedyś mój brat był bardzo cierpliwy, ale teraz jest nerwowy.

So the word order is not completely fixed, but it is chosen here because it sounds natural and clear.

Bardzo means very and intensifies the adjective cierpliwy.

So:

• cierpliwy = patient
• bardzo cierpliwy = very patient

It only modifies the first adjective here, not the second one. That means the sentence says:

• in the past, he was very patient
• now, he is nervous

If you wanted to make the second adjective stronger too, you could say:

• ..., ale teraz jest bardzo nerwowy.

So bardzo works much like English very.