Het paard loopt snel.

In Dutch, every singular noun is either a de-word or a het-word.

Paard (horse) is a het-word, so the correct definite article is het:

• het paard = the horse

You usually have to learn this together with the noun, because it is not always predictable.

A few useful patterns:

• Diminutives always take het: het paardje = the little horse
• Plural nouns take de: de paarden = the horses

So:

• het paard = singular
• de paarden = plural

Loopt is the present-tense form of the verb lopen, which usually means to walk.

The verb changes depending on the subject. With het paard (the horse), the subject is third-person singular, so Dutch uses the -t ending.

Present tense of lopen:

• ik loop = I walk / I am walking
• jij loopt = you walk / are walking
• hij/zij/het loopt = he/she/it walks / is walking
• wij lopen = we walk
• jullie lopen = you all walk
• zij lopen = they walk

So in Het paard loopt snel, loopt agrees with het paard.

It can mean both, depending on context.

Dutch present tense often covers both:

• simple present: walks
• present progressive: is walking

So Het paard loopt snel can mean:

• The horse walks fast
• The horse is walking fast

If Dutch speakers want to emphasize an ongoing action, they can also use another structure, for example:

• Het paard is snel aan het lopen = The horse is walking fast

But the simple sentence Het paard loopt snel is completely normal.

Because snel is describing how the horse walks. In this sentence, it functions like an adverb: quickly / fast.

Dutch word order in a basic main clause is often:

subject + finite verb + rest

So:

• Het paard = subject
• loopt = finite verb
• snel = tells us how it walks

That gives:

• Het paard loopt snel

This is very natural Dutch word order.

Here, snel functions as an adverb, because it describes the verb loopt.

It tells you how the horse walks:

• Het paard loopt snel = The horse walks fast

Compare that with adjective use:

• het snelle paard = the fast horse

There, snelle describes the noun paard, so it is an adjective.

So:

• snel after the verb = adverb use
• snelle before the noun = adjective use

Because in Het paard loopt snel, the word snel is not directly modifying the noun paard. It is modifying the verb loopt.

Dutch adjectives often take an extra -e when they come before a noun:

• het snelle paard = the fast horse

But when the word is used adverbially, it usually stays in the basic form:

• Het paard loopt snel = The horse walks fast

So:

• snelle paard would be part of a noun phrase
• loopt snel is a verb + adverb combination

The difference is what is being described.

• Het paard loopt snel
  = The horse walks fast.
  Here, snel describes the action of walking.

• Het snelle paard loopt
  = The fast horse walks.
  Here, snelle describes the horse itself.

So in the first sentence, the horse may simply be moving quickly right now. In the second, the horse is being identified as a fast horse.

Yes. Vlug is another common word meaning fast / quickly.

So:

• Het paard loopt snel
• Het paard loopt vlug

Both are natural. Snel is probably the most basic and common choice for learners to know first.

Dutch often uses the same word for:

• fast (adjective)
• quickly (adverb)

That is similar to English fast, which can also do both jobs.

The plural of paard is paarden.

So the sentence becomes:

• De paarden lopen snel. = The horses walk fast / are walking fast.

Notice the changes:

• het paard → de paarden
• loopt → lopen

Plural nouns take de, and the verb changes to the plural form.

A rough learner-friendly pronunciation is:

• het ≈ hut or very reduced 't
• paard ≈ paart
• loopt ≈ loapt (with a long o sound, roughly like the vowel in boat, though not exactly the same)
• snel ≈ snel

A very rough full approximation:

• hut paart loapt snel

A few notes:

• The aa in paard is a long vowel.
• The d at the end of paard is often pronounced more like t in actual speech because final voiced consonants are devoiced.
• Dutch oo in loopt is a long vowel.
• het is often unstressed and reduced in normal speech.

If you want to sound natural, stress usually falls on the content words:

• Het PAARD loopt SNEL.