Pchli targ jest dziś otwarty.

Pchli is an adjective meaning flea- or literally of fleas. It comes from pchła (flea), but the form is irregular enough that it does not look very transparent to learners.

So pchli targ is the normal Polish expression for flea market.
Literally, it is something like flea market, just as in English.

A few useful points:

• targ = market
• pchli agrees with targ
• together they form a fixed, common expression: pchli targ

Targ is in the nominative singular.

That is because it is the subject of the sentence:

• Pchli targ = the flea market
• jest dziś otwarty = is open today

So the basic structure is:

• [subject] + [jest] + [adjective]

Since targ is masculine singular nominative, the adjective otwarty is also masculine singular nominative.

Because otwarty must agree with targ in gender, number, and case.

Targ is:

• masculine
• singular
• nominative

So the adjective must also be:

• masculine
• singular
• nominative

That gives otwarty.

Compare:

• sklep jest otwarty = the shop is open
• restauracja jest otwarta = the restaurant is open
• muzeum jest otwarte = the museum is open

In a normal full sentence like this, jest is natural and standard:

• Pchli targ jest dziś otwarty.

Polish can sometimes omit jest, especially in:

• headlines
• notices
• very compressed style
• some colloquial contexts

So you might also see:

• Pchli targ dziś otwarty.

That sounds more like a notice, sign, or headline style.
For a standard learner sentence, jest is the safest and most neutral choice.

Yes, Polish word order is fairly flexible.

This sentence means the same thing in several orders, for example:

• Pchli targ jest dziś otwarty.
• Dziś pchli targ jest otwarty.
• Pchli targ dziś jest otwarty.

The differences are mostly about focus and style, not basic meaning.

The version you have is very natural. It puts the topic first:

• Pchli targ = what we are talking about

Then it adds the information:

• jest dziś otwarty = is open today

If you want to emphasize today, starting with dziś is also very common:

• Dziś pchli targ jest otwarty.

Yes, in this sentence they mean the same thing: today.

You can say:

• Pchli targ jest dziś otwarty.
• Pchli targ jest dzisiaj otwarty.

Both are correct and natural.

Very roughly:

• dziś is shorter and a bit more compact
• dzisiaj is also very common and may sound slightly fuller or more conversational in some contexts

For most learners, it is best to treat them as interchangeable in everyday use.

Yes, pchli is a tricky-looking word for English speakers.

A rough pronunciation guide:

• pch sounds like p
  • the Polish ch sound
• Polish ch sounds like a rough h, like the ch in German Bach or Scottish loch
• li here is a soft l
  • i

Very roughly, pchli sounds like:

• pkh-lee or p-hlee, said quickly

The whole sentence, approximately:

• PCH-lee targ yest jeesh ot-VAR-ty

A few details:

• ch = rough h sound
• j in Polish sounds like English y
• dziś ends in a soft ś sound, somewhat like a soft sh

You do not need a perfect accent right away; the important part is recognizing that pchli begins with a consonant cluster that Polish speakers handle naturally.

Polish has no articles, so there is no separate word for a/an or the.

So Pchli targ can mean:

• a flea market
• the flea market

The context tells you which one is intended.

In this sentence, the natural English translation is often The flea market is open today, because it sounds like a specific market being discussed. But Polish itself does not mark that with an article.

In this sentence, it functions as an adjective meaning open.

Learners may notice that forms like otwarty are historically related to participles, but in everyday grammar here, the easiest and most useful way to think about it is simply:

• otwarty = open

So:

• sklep jest otwarty = the shop is open
• bank jest otwarty = the bank is open
• pchli targ jest otwarty = the flea market is open

That is the practical interpretation you want in sentences like this.

Yes, you can, but it sounds a little different in emphasis.

• Pchli targ jest dziś otwarty. = neutral, very natural
• Pchli targ jest otwarty dziś. = puts more emphasis on today

The final position in Polish often gives a word extra focus. So the second version can feel like:

• The flea market is open today
  maybe implying today, not necessarily on other days

It is correct, but the original sentence is the more neutral default.

Because Polish usually does not need a subject pronoun when the subject noun is already present.

Here the subject is already stated:

• Pchli targ

So there is no need to add anything like on.

In fact, adding on would usually be unnecessary or awkward here, because the noun itself already tells you what the subject is.

Yes. In the plural, the phrase changes form.

Singular:

• pchli targ = flea market
• Pchli targ jest dziś otwarty. = The flea market is open today.

Plural:

• pchle targi = flea markets
• Pchle targi są dziś otwarte. = The flea markets are open today.

Notice the changes:

• pchli targ → pchle targi
• jest → są
• otwarty → otwarte

This is a good example of how Polish changes several words to match number and agreement.