De markt is open op woensdag.

Because markt is a de-word in Dutch. Nouns in Dutch take either de or het as their definite article, and markt happens to take de.

So:

• de markt = the market

There is no simple rule that lets you predict this every time, so learners usually need to memorize whether a noun is a de-word or a het-word.

Dutch main clauses usually follow the V2 rule: the finite verb comes in the second position.

In this sentence:

• De markt = subject
• is = finite verb
• open op woensdag = rest of the sentence

So the order is:

• De markt is open op woensdag.

This is very similar to English The market is open on Wednesday.

Here, open is an adjective.

It describes the subject de markt, and it comes after the verb is:

• De markt is open.

This is called predicative use of the adjective. In Dutch, adjectives used this way usually do not take an extra ending.

Compare:

• de open markt = the open market
• de markt is open = the market is open

Because it comes after the verb is.

In Dutch:

• an adjective before a noun often gets -e
• an adjective after a linking verb like zijn usually does not

So:

• de open markt = adjective before the noun
• de markt is open = adjective after is, so no -e

That difference is very common in Dutch.

Dutch uses op with days of the week.

So:

• op maandag = on Monday
• op dinsdag = on Tuesday
• op woensdag = on Wednesday

This works much like English on Wednesday.

It can often mean either, depending on context.

In a sentence like:

• De markt is open op woensdag.

many people would understand it as a general fact:

• The market is open on Wednesdays / on Wednesday

If you want to make the repeated meaning more obvious, Dutch can also say:

• De markt is elke woensdag open. = The market is open every Wednesday.

So the original sentence often sounds like a schedule statement.

In Dutch, days of the week are normally written with a lowercase letter.

So:

• maandag
• dinsdag
• woensdag

This is different from English, where Wednesday is capitalized.

Yes, that is also possible.

Both of these are natural:

• De markt is open op woensdag.
• De markt is woensdag open.

The first version makes the time phrase more explicit with op.
The second is a bit more compact and is also very common in everyday Dutch.

Dutch allows some flexibility with time expressions, especially in simple sentences like this.

In standard Dutch, woensdag is often pronounced roughly like:

• WOONS-dakh
  or
• WOONS-dahkh

A few helpful notes:

• The w is usually like an English v/w sound, softer than English w
• oe sounds like oo in boot
• The g is the Dutch throat sound, which English does not really have

Learners often notice that the spelling looks longer than the pronunciation feels.

Usually, yes, but the exact meaning depends on context.

markt can mean:

• a marketplace
• an outdoor market
• a town market
• sometimes even the market in an economic sense, depending on context

In this sentence, de markt most likely means a physical market that opens on a certain day.

You would usually add niet:

• De markt is niet open op woensdag. = The market is not open on Wednesday.

Here niet comes after the verb is and before open.

That is the normal way to negate this kind of sentence with zijn + adjective.