Meine Freundin sagt, dass der Privatverkauf heute endet.

Because Freundin is a feminine noun.

In German, possessive words like mein change their endings depending on the gender, case, and number of the noun that follows them.

• mein Freund = my male friend / my boyfriend
• meine Freundin = my female friend / my girlfriend

Here, Freundin is feminine and is the subject of the sentence, so the correct form is meine.

It can mean either one, depending on context.

• meine Freundin often means my girlfriend
• but it can also mean my female friend

German does not always clearly distinguish those two meanings in the noun itself. Context usually tells you which is meant.

Because German normally uses a comma before a subordinate clause, and dass introduces a subordinate clause.

So the sentence is divided like this:

• Main clause: Meine Freundin sagt
• Subordinate clause: dass der Privatverkauf heute endet

That comma is required in standard German.

Because dass sends the conjugated verb to the end of its clause.

This is one of the most important word-order rules in German:

• Main clause: the finite verb usually comes in second position
• dass clause: the finite verb usually goes to the end

So:

• Meine Freundin sagt ... → main clause, verb in second position
• dass der Privatverkauf heute endet → subordinate clause, verb at the end

If you said dass der Privatverkauf endet heute, that would sound wrong in standard German.

German main clauses usually follow the V2 rule, which means the finite verb comes in the second position.

In this sentence:

• Meine Freundin = first element
• sagt = second element

That is standard German word order for a normal statement.

Because Privatverkauf is a masculine noun, and here it is the subject of the subordinate clause.

The basic dictionary form is:

• der Privatverkauf = the private sale

Since it is in the nominative singular, the article stays der.

Privatverkauf means private sale.

It usually refers to a sale by a private person rather than by a business or dealer. For example, someone selling a used item online might describe it as a Privatverkauf.

It is a compound noun:

• privat = private
• Verkauf = sale

German often combines words like this into one noun.

Yes, sometimes.

In the sentence dass der Privatverkauf heute endet, heute is a time expression and sounds very natural there.

You can also say:

• ..., dass heute der Privatverkauf endet.

That version is also correct. It puts a bit more focus on today.

But the verb still stays at the end because of dass.

They are different words.

• dass = that as a conjunction
  
  • Ich weiß, dass... = I know that...
• das = the or that/it as an article or pronoun
  
  • das Haus = the house
  • Das ist gut. = That is good.

In your sentence, dass is correct because it introduces a clause: that the private sale ends today.

Because all German nouns are capitalized.

So in this sentence:

• Freundin is a noun
• Privatverkauf is a noun

That is why they begin with capital letters.

endet is the third-person singular present tense of enden.

The subject is der Privatverkauf, which is singular, so German uses:

• ich ende
• du endest
• er/sie/es endet

So der Privatverkauf heute endet means the private sale ends today.

Yes, that is possible in German.

• Meine Freundin sagt, dass der Privatverkauf heute endet.
• Meine Freundin sagt, der Privatverkauf endet heute.

Both are grammatical.

The version with dass is very explicit and common.
The version without dass is a little more direct and keeps normal main-clause word order in the second clause.

Yes.

German often uses the present tense to talk about the near future, especially when there is a time word like heute, morgen, or nächste Woche.

So endet heute literally uses the present tense, but in English you might translate it as either:

• ends today
• is ending today

depending on context.