Het konijn loopt in de tuin.

Because konijn is a het-word in Dutch. Dutch singular nouns normally take either de or het, and you usually have to learn this with the noun:

• het konijn = the rabbit
• de tuin = the garden

There are some patterns in Dutch, but for many nouns the article just has to be memorized.

A useful thing to remember: in the plural, the article is always de:

• het konijn = the rabbit
• de konijnen = the rabbits

Because loopt is the third-person singular present-tense form of lopen.

The verb lopen = to walk / to move on foot.

Present tense:

• ik loop = I walk / I am walking
• jij loopt = you walk
• hij/zij/het loopt = he/she/it walks
• het konijn loopt = the rabbit walks / is walking

So het konijn behaves like het or it, which takes loopt.

It can mean either, depending on context.

Dutch often uses the simple present where English might use either:

• walks
• is walking

So Het konijn loopt in de tuin can mean:

• The rabbit walks in the garden
• The rabbit is walking in the garden

Dutch does have a way to stress an ongoing action, for example:

• Het konijn is in de tuin aan het lopen

But in many everyday sentences, the simple present is enough.

This is the normal word order for a simple main clause in Dutch:

subject + finite verb + other information

So:

• Het konijn = subject
• loopt = finite verb
• in de tuin = prepositional phrase telling where

This is very similar to basic English word order:

• The rabbit
  • is walking
    • in the garden

Because tuin is a de-word:

• de tuin = the garden

The preposition in does not change the article here. So you simply keep the normal article of the noun:

• in de tuin = in the garden

Unlike German, Dutch does not have a big case system that changes the article in this kind of sentence.

Here in means in or inside/within an area.

With places like de tuin, Dutch often uses in where English might say:

• in the garden
• sometimes even in the yard

So in de tuin means the rabbit is located within that garden area.

In this sentence, het is an article, not a pronoun.

So:

• het konijn = the rabbit

Dutch het can also mean it in other sentences, for example:

• Het loopt in de tuin. = It is walking in the garden.

But in Het konijn loopt in de tuin, het is just the word the before a neuter noun.

A rough pronunciation guide:

• het ≈ ut or hut with a very weak vowel
• konijn ≈ ko-NINE
• loopt ≈ loapt with a long o
• in = similar to English in
• de ≈ duh with a weak vowel
• tuin is tricky, roughly like tow-n but with a Dutch vowel not found exactly in English

A rough full approximation:

• ut ko-NINE loapt in duh tow-n

This is only approximate, but it can help at the start.

Not always exactly. Tuin often corresponds to garden, but depending on context it can also be closer to yard.

For example:

• a space behind a house with plants, grass, or a patio can be a tuin
• in some English contexts, especially American English, you might naturally say yard

So de tuin is often translated as the garden, but the real-life meaning may overlap with yard.

You would change both the noun and the verb:

• Het konijn loopt in de tuin. = The rabbit is walking in the garden.
• De konijnen lopen in de tuin. = The rabbits are walking in the garden.

Notice the changes:

• het konijn → de konijnen
• loopt → lopen

This shows two useful Dutch rules:

1. plural nouns take de
2. plural subjects use the plural verb form

Yes, lopen is fine here. It simply describes the rabbit moving around on foot.

But Dutch could use other verbs depending on the exact action:

• lopen = walk / move around
• rennen = run
• springen = jump

So:

• Het konijn loopt in de tuin = the rabbit is walking / moving around in the garden
• Het konijn rent in de tuin = the rabbit is running in the garden
• Het konijn springt in de tuin = the rabbit is jumping in the garden

In other words, lopen is natural, but it is not the only possible verb.