Die kleine Hündin bellt, wenn jemand an der Tür steht.

Because Hündin is a feminine singular noun in the nominative case, and it is the subject of the sentence.

• die = feminine singular the
• kleine = adjective small/little with the correct ending after die
• Hündin = female dog

So:

• die kleine Hündin = the small female dog

German articles and adjective endings change depending on gender, number, and case, so this whole phrase is built to match a feminine nominative singular noun.

• der Hund = dog (can sometimes mean a dog in general, or specifically a male dog depending on context)
• die Hündin = female dog

So Hündin is specifically feminine. English learners often notice that the direct equivalent can sound stronger in English, but in German Hündin is a normal animal term.

Because in German, adjectives placed before nouns usually take an ending.

Here, klein becomes kleine because it comes after the definite article die and describes a feminine singular nominative noun:

• die kleine Hündin

This is called an adjective ending. After der/die/das, the adjective often takes a weaker ending, and for feminine nominative singular that ending is -e.

The verb is bellen = to bark.

In the present tense:

• ich belle
• du bellst
• er/sie/es bellt
• wir bellen
• ihr bellt
• sie/Sie bellen

The subject here is die kleine Hündin, which is third person singular, so the correct form is bellt.

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

Main clause:

• Die kleine Hündin bellt

Subordinate clause:

• wenn jemand an der Tür steht

This is a very common German word-order rule:

• ..., wenn er kommt
• ..., weil sie müde ist
• ..., dass ich keine Zeit habe

So the final position of steht is completely normal.

Here wenn means when in the sense of whenever / if.

So the sentence means something like:

• the little dog barks when/whenever someone is at the door

This is not the same as wann, which is used in direct or indirect questions:

• Wann kommt er? = When is he coming?

Also compare:

• als = when for a single event in the past
• wenn = when/whenever/if for repeated events, general situations, present/future situations, or conditions

In this dog sentence, wenn fits because it describes a general repeated situation.

Jemand means someone / somebody.

That is the most natural choice here:

• wenn jemand an der Tür steht = when someone is at the door

Irgendjemand means someone or other / just anybody / some random person, which is a bit more indefinite or emphatic.

So:

• jemand = neutral, normal
• irgendjemand = more vague or more emphatic

In this sentence, jemand is the simple and natural option.

Because an is the normal preposition here for being at the door.

• an der Tür stehen = to be standing at the door

This is a location, not movement, so an takes the dative case here.

That gives:

• die Tür (nominative)
• an der Tür (dative after an for location)

If there were movement toward the door, you could get the accusative:

• Er geht an die Tür. = He goes to the door.

But here the person is already there, so:

• jemand steht an der Tür

Because der here is not masculine nominative. It is the dative feminine singular form of the definite article.

The article die changes by case:

• nominative: die Tür
• accusative: die Tür
• dative: der Tür
• genitive: der Tür

Since an is being used for location, it takes the dative here:

• an der Tür

So der is correct because it is dative feminine, not because the noun is masculine.

Yes, literally steht means stands / is standing.

But in this sentence, jemand an der Tür steht is often best understood naturally as:

• someone is at the door
• someone is standing at the door

German often uses stehen where English might prefer a more general expression like be at the door.

So the literal meaning is is standing, but the natural meaning may be slightly broader.

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

So:

• Die kleine Hündin bellt, wenn jemand an der Tür steht.

This comma is required in standard German.

English punctuation is often lighter in similar sentences, but German is stricter about marking subordinate clauses with commas.