Dzisiejszy bieg w parku był krótki.

Dzisiejszy is the adjective meaning today’s or of today. It has the ending -y because it agrees with bieg, which is:

• masculine
• singular
• nominative (the subject of the sentence)

In Polish, adjectives must match the noun they describe in gender, number, and case.

So:

• dzisiejszy bieg = today’s run

If the noun were feminine, neuter, or plural, the adjective would change form.

Bieg is in the nominative case because it is the subject of the sentence.

The basic structure is:

• Dzisiejszy bieg w parku = the subject
• był krótki = what is being said about the subject

So the sentence literally works like:

• Today’s run in the park was short

After the preposition w meaning in, Polish usually uses the locative case when talking about location.

So:

• park = base form / nominative
• w parku = in the park

This is a very common pattern:

• w domu = in the house
• w sklepie = in the shop
• w lesie = in the forest

Here, w parku tells you where the run took place.

Był is the past tense form of być meaning to be.

It is used because the sentence is talking about something in the past:

• był = was

It also agrees with the subject bieg, which is masculine singular.

Compare:

• bieg był krótki = the run was short
• gra była krótka = the game was short
• spotkanie było krótkie = the meeting was short

So Polish past tense forms of to be reflect gender in the singular.

Krótki is an adjective, and here it describes the noun bieg.

• krótki = short

You need an adjective because you are describing what the run was like.

Krótko is an adverb, meaning briefly / for a short time, and it would describe a verb, not a noun.

So:

• Bieg był krótki = The run was short
• Biegliśmy krótko = We ran for a short time / briefly

Because both are adjectives connected to the masculine singular noun bieg.

• dzisiejszy describes which run
• krótki describes what kind of run it was

Both adjectives agree with bieg, so both appear in masculine singular forms.

Even though krótki comes after był, it still matches the subject.

Yes, in standard modern Polish, after być the predicate adjective is normally in the nominative.

So:

• bieg był krótki

Both bieg and krótki are masculine singular nominative.

This is similar to saying:

• Obiad był smaczny = The lunch was tasty
• Film był ciekawy = The film was interesting

Polish does not have articles like the or a/an.

So a noun like bieg can mean:

• a run
• the run

The exact meaning depends on context.

In this sentence, English would naturally say Today’s run in the park was short, but Polish simply says:

• Dzisiejszy bieg w parku był krótki

without any article.

Yes. Bieg can refer to:

• a run as an activity
• a race, depending on context

In this sentence, if the meaning shown to the learner is run, then that is the intended meaning. But in another context, bieg w parku could also sound like a race in the park.

If you want to stress the activity of running in a more general way, Polish often uses bieganie:

• Dzisiejsze bieganie w parku było krótkie = Today’s running in the park was short

But bieg is perfectly natural for run in many contexts.

It is much more natural to say:

• Dzisiejszy bieg w parku był krótki

because w parku stays close to bieg, the thing it modifies.

If you move w parku to the end:

• Dzisiejszy bieg był krótki w parku

it can sound less natural or slightly confusing, as if short in the park were being treated as one unit.

Polish word order is flexible, but not every order sounds equally natural.

Yes, very often they do.

So:

• dzisiejszy bieg
• krótki spacer
• duży dom

Putting the adjective after the noun is sometimes possible, but it usually sounds:

• more formal
• more literary
• more marked for emphasis

So for a normal everyday sentence, dzisiejszy bieg is the expected order.

The adjective and the past tense form of to be would change to match the noun.

For example:

• Dzisiejsza gra w parku była krótka
  game is feminine

• Dzisiejsze spotkanie w parku było krótkie
  meeting is neuter

So in your original sentence:

• dzisiejszy
• był
• krótki

all show agreement with the masculine noun bieg.

Yes, it is natural and grammatical.

It sounds like a normal way to say that today’s run in the park was short. The structure is straightforward and common:

• adjective + noun
• place phrase
• past tense of to be
• adjective describing the subject

So it is a good model sentence for learning:

• adjective agreement
• the locative after w
• past tense gender agreement
• basic Polish sentence structure