Min nabo sælger et gammelt møbel på loppemarkedet på lørdag.

Because nabo is a common-gender noun in Danish. Common-gender nouns take en in the indefinite form and use possessives like min, din, sin, etc.

• en nabo = a neighbor
• min nabo = my neighbor

If the noun were neuter (a et noun), you would use mit instead:

• et hus = a house
• mit hus = my house

Because møbel is a neuter noun, so it takes et:

• et møbel = a piece of furniture

In Danish, adjectives usually agree with the gender and number of the noun.

So:

• common gender singular: en gammel stol
• neuter singular: et gammelt møbel
• plural: gamle møbler

That extra -t on gammelt shows that the noun is neuter.

This is adjective agreement.

The basic form is often shown as the common-gender singular form:

• gammel = old

But before a neuter singular noun, the adjective usually adds -t:

• en gammel bog = an old book
• et gammelt møbel = an old piece of furniture

Before plurals or definite forms, you usually use gamle:

• gamle møbler = old furniture / old pieces of furniture
• det gamle møbel = the old piece of furniture

That -et is the definite ending. Danish often puts the at the end of the noun instead of using a separate word like English does.

• et loppemarked = a flea market
• loppemarkedet = the flea market

So på loppemarkedet means at the flea market.

This is very common in Danish:

• en bil → bilen
• et hus → huset
• et møbel → møblet

Because på is used in both expressions, but for different reasons.

1. på loppemarkedet = at the flea market
   Here på is a preposition of place.

2. på lørdag = on Saturday / this Saturday
   Here på is used in a time expression.

English speakers often expect different prepositions to match exactly between English and Danish, but they often do not. In Danish, på is very common in both place and time expressions.

Usually it means this coming Saturday, depending on context.

So:

• på lørdag = on Saturday / this Saturday

If the context is clear, Danish often uses this expression for the next Saturday coming up.

Danish often uses the present tense to talk about the future when the time is already clear from context.

Here, på lørdag makes the future meaning obvious:

• Min nabo sælger et gammelt møbel på loppemarkedet på lørdag.

Even though sælger is grammatically present tense, the sentence refers to a future event because of på lørdag.

English does this too sometimes:

• My neighbor is selling it on Saturday
• We leave tomorrow

So this is very natural in Danish.

The infinitive is at sælge = to sell.

Some useful forms are:

• at sælge = to sell
• sælger = sell / is selling
• solgte = sold
• har solgt = has sold

In this sentence, sælger is the present tense form.

This sentence has the normal main-clause Danish word order:

Min nabo + sælger + et gammelt møbel + på loppemarkedet + på lørdag

So:

• subject: Min nabo
• verb: sælger
• object: et gammelt møbel
• place: på loppemarkedet
• time: på lørdag

A very important feature of Danish is that in a main clause, the finite verb usually comes in second position. Here that happens naturally because the sentence begins with the subject.

If you started with the time expression, the verb would still stay in second position:

• På lørdag sælger min nabo et gammelt møbel på loppemarkedet.

Notice that sælger still comes right after the first element.

Yes, that is possible, but the original order sounds more natural if you want to keep place before time in a neutral statement.

Both are understandable:

• ... på loppemarkedet på lørdag
• ... på lørdag på loppemarkedet

In practice, Danish word order with adverbials can vary depending on what you want to emphasize. The original sentence is a very natural, straightforward version.

It can be any gender. The word nabo just means neighbor.

If needed, Danish can specify:

• mandlig nabo = male neighbor
• kvindelig nabo = female neighbor

But normally nabo is neutral in meaning, like English neighbor.

A few parts may be difficult for English speakers:

• min: fairly close to meen
• nabo: roughly NA-bo
• sælger: the æ is an open front vowel, somewhat like the vowel in English cat, but not exactly
• møbel: the ø has no exact English equivalent; it is a rounded front vowel
• lørdag: the ø is again tricky, and the g at the end is not pronounced like a strong English g
• loppemarkedet: long word, but it breaks down as loppe + marked + et

For many learners, æ, ø, and Danish soft consonants are harder than the grammar here.

Marked means market.
Loppemarked literally means flea market.

It is a compound noun:

• loppe = flea
• marked = market
• loppemarked = flea market

Then the definite form is:

• loppemarkedet = the flea market

Compound nouns are very common in Danish, just as in German and often in English too.