Op de markt koopt mijn moeder een komkommer, en ik koop een grote ui.

Because Dutch uses verb-second word order in main clauses.

In this sentence, Op de markt is placed first for emphasis or topic. When something other than the subject comes first, the finite verb must still stay in the second position, so the subject moves after it:

• Mijn moeder koopt een komkommer op de markt.
• Op de markt koopt mijn moeder een komkommer.

Both are correct. The second version emphasizes where the action happens.

In the second clause, the subject ik comes first, so normal Dutch main-clause order is used:

• ik koop

Dutch main clauses usually follow this pattern:

• first position: subject or some other element
• second position: finite verb

So:

• Ik koop een grote ui. → subject first, then verb
• Op de markt koop ik een grote ui. → place first, then verb, then subject

In your sentence, the second clause starts with ik, so there is no inversion.

Op de markt means at the market or on the market square / marketplace.

Dutch often uses op with places that are seen as open areas, surfaces, or public event spaces. A market is often conceptualized that way, so op de markt is the normal expression.

Compare:

• op school = at school
• op straat = in the street / on the street
• op de markt = at the market

English and Dutch do not always use the same preposition, so it is best to learn this as a fixed phrase.

Yes, that is grammatical.

Dutch allows some flexibility in where adverbial phrases like op de markt go. These are all possible:

• Mijn moeder koopt een komkommer op de markt.
• Mijn moeder koopt op de markt een komkommer.
• Op de markt koopt mijn moeder een komkommer.

The version in your sentence puts op de markt first, which gives it more emphasis and also creates the inversion koopt mijn moeder.

Because in Dutch, adjectives usually take an -e ending before a noun.

So:

• een grote ui
• een mooie appel
• de grote ui

For learners, a useful basic rule is:

• adjectives usually get -e
• one important exception: with an indefinite article (een) + a singular neuter noun, the adjective often has no -e

For example:

• een groot huis (huis is neuter, a het-word)

But ui is a de-word, not a het-word, so you say:

• een grote ui

You mostly have to learn noun gender together with the noun.

The singular is:

• de ui

So it is a common-gender noun (often called a de-word). That is why the adjective in een grote ui takes -e.

Its plural is:

• de uien

Plural nouns always take de in Dutch.

Een is the indefinite article, like a or an in English.

So:

• een komkommer = a cucumber
• een grote ui = a big onion

It is used because the sentence is talking about one unspecified item of each kind, not a specific one already known to the listener.

If the items were specific, Dutch might use de or het instead, depending on the noun:

• de komkommer
• de grote ui

Because the verb changes according to the subject.

The infinitive is kopen = to buy.

Present tense:

• ik koop = I buy
• jij/je koopt = you buy
• hij/zij koopt = he/she buys
• mijn moeder koopt = my mother buys
• wij kopen = we buy

So:

• mijn moeder koopt because mijn moeder is third-person singular
• ik koop because ik takes the form without -t

Yes. In this sentence, en simply joins two main clauses:

• Op de markt koopt mijn moeder een komkommer
• ik koop een grote ui

So en means and.

The comma before en is acceptable here because two full clauses are being linked. In everyday writing, punctuation around en can vary a little, but the meaning stays the same.

Yes, ui is one of the trickier Dutch vowel combinations for English speakers.

It is pronounced roughly like the Dutch diphthong in words such as:

• huis
• muis
• bruin

There is no perfect English equivalent. It is something like a vowel moving from a sound a bit like uh toward something rounded.

The important point is that ui is a single vowel sound combination in Dutch, not two separate vowels pronounced one after the other.

Yes. Koopt and koop are both present tense.

Like the English present tense, Dutch present tense can describe:

• something happening now
• a habitual action
• a general fact
• a vivid narrative present

So this sentence could mean:

• right now, at the market, my mother is buying a cucumber and I am buying a big onion
• or more generally, that is what happens in this situation

The exact interpretation depends on context.

Because it is often unnecessary. Once the location has been established, Dutch can leave it out if it clearly applies to both clauses.

So the sentence naturally suggests:

• At the market, my mother buys a cucumber, and I buy a big onion.

If you wanted to repeat it, you could say:

• Op de markt koopt mijn moeder een komkommer, en op de markt koop ik een grote ui.

But that sounds repetitive unless you really want to stress the location again.