Unsere Katze schaut aus dem Fenster, wenn kleine Tiere an der Wand krabbeln.

Because Katze is a feminine noun in German: die Katze.

The possessive unser- has to match the gender, number, and case of the noun that follows it. Here, Katze is:

• feminine
• singular
• in the nominative case because it is the subject of the sentence

So the correct form is unsere Katze.

Compare:

• unser Hund = our dog
• unsere Katze = our cat
• unser Kind = our child
• unsere Kinder = our children

In a normal German main clause, the finite verb usually comes in position 2.

The sentence begins with the subject:

• Unsere Katze = subject
• schaut = finite verb

So the structure is:

• Unsere Katze / schaut / aus dem Fenster

This is a basic German word order pattern:

• Subject + verb + other information

Even if something else comes first, the verb still stays in second position. For example:

• Aus dem Fenster schaut unsere Katze.

That sounds more marked or literary, but the rule is the same: the verb is still second.

Here, schaut aus belongs together as the verb ausschauen or, in everyday usage, aus etwas schauen in the sense of to look out of something.

So:

• schauen = to look
• aus dem Fenster schauen = to look out of the window

In this sentence, aus is closely tied to the idea of looking outward through the window.

Be careful: aus can also be a normal preposition in many other contexts, such as:

• aus Berlin = from Berlin
• aus Holz = made of wood

Here, though, learners will often understand aus dem Fenster schauen as a fixed expression meaning to look out the window.

Because the preposition aus always takes the dative case.

The noun is das Fenster, but after aus it becomes dem Fenster:

• das Fenster = nominative / accusative
• dem Fenster = dative

So:

• aus dem Fenster = out of the window / out the window

This is something you simply have to learn with the preposition:

• aus + dative
• mit + dative
• von + dative
• zu + dative

Because wenn introduces a subordinate clause, and in German subordinate clauses, the finite verb goes to the end.

So:

• wenn = when / whenever / if
• kleine Tiere = small animals
• an der Wand = on the wall
• krabbeln = crawl

The structure is:

• wenn + subject + other information + verb

That is why you get:

• wenn kleine Tiere an der Wand krabbeln

and not:

• wenn kleine Tiere krabbeln an der Wand

Here, wenn means something like when or whenever.

The sentence describes a repeated or habitual situation:

• the cat looks out the window whenever small animals crawl on the wall

So in this sentence, wenn is not really a one-time future when, but more of a recurring whenever.

German wenn can mean:

• when for repeated events
• whenever
• sometimes if

Examples:

• Wenn ich müde bin, gehe ich früh ins Bett.
  = When/Whenever I am tired, I go to bed early.

• Wenn es regnet, bleiben wir zu Hause.
  = If it rains, we stay home.

Because an is a two-way preposition. That means it can take either:

• accusative for direction/movement toward
• dative for location/position

Here, the animals are already crawling on the wall, so the wall is the location, not the destination. That is why German uses the dative:

• an der Wand = on the wall

Compare:

• Die Fliege krabbelt an der Wand.
  = The fly is crawling on the wall.

• Die Fliege fliegt an die Wand.
  = The fly flies onto the wall.

So:

• where? → dative
• where to? → accusative

Krabbeln usually means to crawl, to creep, or to scuttle, especially for small animals, insects, or babies.

In this sentence, it suggests the movement of tiny creatures on the wall, such as:

• insects
• spiders
• other little creeping animals

It gives a more specific picture than a general verb like bewegen (to move).

Some related verbs are:

• kriechen = to crawl, creep
• laufen = to run / walk
• klettern = to climb

Krabbeln often sounds small, busy, and a little fidgety.

Yes, but it would mean something slightly different.

• aus dem Fenster schauen = to look out the window
• aus dem Fenster sehen can also occur, but sehen is more about seeing, while schauen is more about actively looking

So:

• Die Katze schaut aus dem Fenster.
  = The cat is looking out the window.

• Die Katze sieht ein Tier.
  = The cat sees an animal.

In many contexts, schauen is the more natural choice when talking about directing your gaze somewhere.

Because in German, a subordinate clause introduced by a word like wenn is normally separated from the main clause with a comma.

So the comma marks the boundary between:

• Unsere Katze schaut aus dem Fenster = main clause
• wenn kleine Tiere an der Wand krabbeln = subordinate clause

This is standard German punctuation.

You will also see commas before subordinate clauses introduced by words like:

• weil = because
• dass = that
• obwohl = although
• wenn = when/if

Yes. German allows that very naturally.

You can also say:

• Wenn kleine Tiere an der Wand krabbeln, schaut unsere Katze aus dem Fenster.

That means the same thing.

But notice the word order in the main clause after the subordinate clause. German does not say:

• Wenn kleine Tiere an der Wand krabbeln, unsere Katze schaut aus dem Fenster.

Instead, after the introductory subordinate clause, the finite verb comes immediately:

• ..., schaut unsere Katze ...

This is because the whole subordinate clause takes up position 1, so the main verb must come in position 2.

It most naturally sounds like a general habit or repeated behavior.

That comes from a few things:

• the present tense schaut
• the use of wenn
• the lack of any time expression pointing to one specific moment

So the idea is:

• Our cat looks out the window whenever small animals crawl on the wall.

It describes what the cat typically does in that situation.

If you wanted a single event in the past, German would usually use a different tense and clearer context.