Der Hund wacht vor dem Haus.

Because Der Hund is the subject of the sentence — the dog is the one doing the action.

• der = masculine nominative singular
• den = masculine accusative singular

Since the dog is the one that wacht (keeps watch / is awake), German uses the nominative: Der Hund.

wacht is the 3rd person singular form of wachen.

Here, wachen means:

• to stay awake
• to keep watch
• to stand guard

So this is not the normal verb for watching something like TV. For that, German usually uses verbs like sehen, anschauen, or gucken/schauen depending on context.

They are historically related, and the meanings overlap a little, but you should not assume they work the same way.

In this sentence, wacht means something like:

• is on guard
• keeps watch
• is awake

It does not usually mean is watching in the everyday English sense, like watching a movie.

Because vor is a two-way preposition.

Two-way prepositions take:

• dative for location: where something is
• accusative for direction/movement toward somewhere

Here, the dog is located in front of the house, so German uses the dative:

• vor dem Haus = in front of the house

If there were movement toward that position, you could use accusative:

• Der Hund läuft vor das Haus. = The dog runs to a position in front of the house.

Because das is the nominative/accusative article for a neuter noun, but after vor in a location meaning, the noun must be in the dative.

For das Haus, the dative form is:

• das Haus → dem Haus

So:

• das Haus = the house
• vor dem Haus = in front of the house

This is a normal German main clause, where the conjugated verb comes in second position:

• Der Hund = first element
• wacht = second element
• vor dem Haus = the rest

So the pattern is:

Subject + verb + prepositional phrase

You could also move the location to the front:

• Vor dem Haus wacht der Hund.

That is still correct, because wacht remains in second position.

It can suggest that idea, but it does not literally say the dog guards the house.

Der Hund wacht vor dem Haus means more literally that the dog keeps watch or stands guard in front of the house.

If you want to say more directly The dog guards the house, German might use:

• Der Hund bewacht das Haus.

That verb focuses more clearly on guarding the house itself.

Because in German, all nouns are capitalized.

So:

• der Hund
• das Haus

This is one of the big visual differences between German and English spelling.

It could be either, depending on context.

German simple present often covers both:

• something happening right now
• something that happens regularly / generally

So Der Hund wacht vor dem Haus could mean:

• the dog is keeping watch there now, or
• the dog generally stands guard there

Context tells you which meaning is intended.

A rough English-style guide is vahkht.

A few important points:

• w in German sounds like English v
• a is a short ah
• ch after a is the rough sound heard in Bach
• the t at the end is pronounced

So wacht is not pronounced like English watched.

The most basic negation is:

• Der Hund wacht nicht vor dem Haus.

But where you put nicht changes the emphasis.

For example:

• Der Hund wacht nicht vor dem Haus.
  = The dog is not keeping watch in front of the house / The dog does not keep watch in front of the house.

• Der Hund wacht nicht vor dem Haus, sondern hinter dem Haus.
  = The dog is not in front of the house, but behind it.

So nicht can negate the whole statement or specifically the location, depending on context.