Die Kinder grinsen, weil der Hund im Garten laut gähnt.

Because Kinder is plural, and in the nominative plural the definite article is die.

• das Kind = the child
• die Kinder = the children

In this sentence, Die Kinder is the subject of grinsen, so it is nominative plural.

Also, Kinder is one of those German plurals that does not add an extra ending beyond -er:

• das Kind
• die Kinder

Because weil introduces a subordinate clause, and in German subordinate clauses are normally separated from the main clause by a comma.

So the sentence is divided like this:

• Die Kinder grinsen = main clause
• weil der Hund im Garten laut gähnt = subordinate clause

German is much stricter about this than English. In a sentence with weil, the comma is required.

Because weil sends the conjugated verb to the end of the subordinate clause.

Compare:

• Main clause: Der Hund gähnt laut.
• Subordinate clause: ..., weil der Hund laut gähnt.

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

• main clause → conjugated verb usually in second position
• subordinate clause with words like weil, dass, wenn, obwohl → conjugated verb usually at the end

So weil der Hund im Garten laut gähnt is normal German word order.

Because der Hund is the subject of the subordinate clause, so it must be in the nominative case.

Inside the weil clause, ask: who is yawning?

• der Hund is yawning

That makes der Hund the subject.

Quick comparison:

• der Hund = nominative, subject
• den Hund = accusative, direct object

So here:

• weil der Hund ... gähnt = because the dog is yawning

im is a contraction of in dem.

• in dem Garten → im Garten

This is very common in German.

Here it means in the garden and shows location, not movement. Because it describes where the dog is, in takes the dative here.

So:

• in den Garten = into the garden, movement toward
• im Garten = in the garden, location

That is why the sentence uses im Garten.

laut is an adverb here, modifying gähnt. It tells us how the dog yawns.

In a subordinate clause, the verb goes to the end, so adverbs often appear before that final verb:

• weil der Hund im Garten laut gähnt

This sounds natural.

You could sometimes rearrange adverbs in German for emphasis, but the version in the sentence is very normal and neutral.

Because here laut is being used as an adverb, not an adjective directly in front of a noun.

Compare:

• der laute Hund = the loud dog
  
  • here laut is an adjective and gets an ending: laute
• der Hund gähnt laut = the dog yawns loudly
  
  • here laut is an adverb, so it stays laut

German adverbs usually do not take endings.

Both can describe a happy facial expression, but they are not quite the same.

• lächeln = to smile
• grinsen = to grin

grinsen often suggests:

• a broader smile
• a cheeky, amused, or mischievous expression
• sometimes even a slightly silly or smug look

So Die Kinder grinsen feels a bit stronger or more playful than Die Kinder lächeln.

The infinitive is gähnen = to yawn.

gähnt is the third-person singular present tense form:

• ich gähne
• du gähnst
• er/sie/es gähnt

So in the sentence:

• der Hund ... gähnt = the dog yawns / is yawning

German present tense can often translate into either simple present or present progressive in English, depending on context.

The tricky part is the ä.

A rough guide:

• gäh- sounds somewhat like the e in British care or air, but shorter and more German
• -nt is pronounced clearly at the end

So gähnt is approximately like gehnt, but with the fronted vowel ä, not a plain English a.

A few helpful points:

• ä is not the same as a
• h after the vowel shows that the vowel is long
• gähnen and gähnt should have a long vowel sound

Yes, that is possible, but it changes the feel slightly.

• weil der Hund im Garten laut gähnt = neutral; location first, then manner
• weil der Hund laut im Garten gähnt = puts a bit more focus on laut

German allows some flexibility with adverb order, especially with elements like place and manner. But the original sentence sounds very natural and standard.

If you are learning, im Garten laut gähnt is a good pattern to remember.

Because German often uses the present tense to describe something happening now or generally true in the situation.

• Die Kinder grinsen can mean:
  • the children are grinning
  • the children grin

Likewise:

• der Hund ... gähnt can mean:
  • the dog is yawning
  • the dog yawns

German does not need a separate form like English are grinning or is yawning. The simple present often covers both ideas.

Yes. You can also say:

• Weil der Hund im Garten laut gähnt, grinsen die Kinder.

That is completely correct.

Notice what happens in the main clause after the subordinate clause:

• grinsen comes before die Kinder

That is because the whole weil clause takes the first position, so the conjugated verb in the main clause must still stay in second position.

This is a very common German pattern:

• Weil ..., verb subject ...