Kleine Tiere krabbeln auf dem Balkon herum, wenn es warm ist.

Because Tiere is the subject of the sentence, so it is in the nominative plural.

With no article before the noun, the adjective klein- takes the strong ending -e in the nominative plural:

• kleine Tiere = small animals

So kleine Tiere is the correct form here.

Because the subject is plural: Tiere = animals.

German verbs must agree with the subject:

• das Tier krabbelt = the animal crawls
• die Tiere krabbeln = the animals crawl

So krabbeln is the correct plural form.

Herum adds the idea of around or about.

So krabbeln herum means something like:

• crawl around
• scuttle about

It suggests movement with no fixed destination.

You can think of herumkrabbeln as a separable verb:

• infinitive: herumkrabbeln
• in a normal main clause: krabbeln ... herum

That is why herum appears later in the sentence instead of right next to the verb.

Because auf is a two-way preposition. It can take:

• dative for location: where something is
• accusative for direction: where something is going

Here the animals are already on the balcony, crawling around there. That is location, so German uses the dative:

• auf dem Balkon = on the balcony

If the meaning were movement onto the balcony, you would use accusative:

• auf den Balkon = onto the balcony

Because wenn es warm ist is a subordinate clause, and German normally separates subordinate clauses from the main clause with a comma.

So:

• Kleine Tiere krabbeln auf dem Balkon herum, wenn es warm ist.

That comma is required in standard German.

Because wenn introduces a subordinate clause.

In German subordinate clauses, the conjugated verb goes to the end. So:

• es ist warm = it is warm
• wenn es warm ist = when it is warm

This is one of the most important word-order patterns in German.

Here es is an impersonal or dummy subject. It does not refer to a specific noun like Tier or Balkon.

German often uses es for weather, temperature, and general conditions:

• Es ist warm. = It is warm.
• Es regnet. = It is raining.
• Es ist kalt. = It is cold.

So in this sentence, es just means it in the general English sense.

Because warm comes after sein and functions as a predicate adjective.

In German, predicate adjectives do not take adjective endings:

• Das Tier ist klein.
• Die Tiere sind klein.
• Es ist warm.

So warm stays unchanged.

Here wenn means when or whenever.

German wenn can sometimes mean:

• when = in time
• if = condition

In this sentence, the meaning is more like a general repeated situation:

• when/whenever it is warm

So the sentence has a habitual sense: small animals crawl around on the balcony whenever the weather is warm.

This is a very common question.

• wenn is used for:

  • repeated events in the past
  • present and future situations
  • general truths
  • conditions

• als is used for:

  • a single completed event in the past

So here wenn is correct because the sentence describes a general recurring situation, not one single past event.

Compare:

• Wenn es warm ist, ... = when/whenever it is warm, ...
• Als es gestern warm war, ... = when it was warm yesterday, ...

Because in German, all nouns are capitalized.

So:

• Tiere = noun
• Balkon = noun

But kleine is an adjective, so it is normally written with a lowercase letter.

This is a standard spelling rule in German.

Yes. That version is also correct.

German allows either order:

• Kleine Tiere krabbeln auf dem Balkon herum, wenn es warm ist.
• Wenn es warm ist, krabbeln kleine Tiere auf dem Balkon herum.

If the wenn clause comes first, the main clause still keeps the verb in the normal second-position slot:

• Wenn es warm ist, krabbeln ...

So the clause order can change, but the grammar rules stay the same.