De tuinvereniging heeft een tuinreglement.

Dutch very often forms compound nouns by joining words together into a single word.

• tuin = garden

• vereniging = association / club
  → tuinvereniging = garden association / gardening club

• tuin = garden
• reglement = regulations / rulebook
  → tuinreglement = garden regulations / garden rulebook

This is much more common in Dutch than in English. Where English often uses two words, Dutch often prefers one compound word.

De is the definite article, meaning the.

So:

• de tuinvereniging = the garden association

In Dutch, nouns have common gender or neuter gender. Vereniging is a de-word, so it takes de, not het.

Een means a or an.

So:

• een tuinreglement = a garden regulation set / a garden rulebook

The sentence is talking about a set of regulations, not a specific one already identified in the sentence. That is why een is used.

Also, reglement is a het-word in Dutch: het reglement. But after een, you do not see the gender difference, because both de-words and het-words use een in the singular indefinite form.

Because the subject is singular:

• de tuinvereniging = the garden association

For the verb hebben (to have), the present tense goes like this:

• ik heb
• jij/u hebt or heeft
• hij/zij/het heeft
• wij/jullie/zij hebben

Since de tuinvereniging is a singular noun, Dutch uses heeft.

Yes, in this sentence it is very similar to English:

• De tuinvereniging = subject
• heeft = verb
• een tuinreglement = object

So the structure is:

Subject + Verb + Object

That is the normal main-clause word order in simple Dutch statements.

No. Just like English has, Dutch heeft can mean more than owning something physically.

Here, heeft een tuinreglement means something like:

• has regulations
• has a set of rules
• has a rulebook

So it expresses that the association possesses or maintains an official set of rules.

Vereniging means association, club, or organized group.

It comes from the verb verenigen, meaning to unite or to bring together.

In everyday Dutch, vereniging is very common for clubs and member organizations, such as:

• sportvereniging = sports club
• muziekvereniging = music association
• tuinvereniging = garden association

The usual plurals are:

• de tuinvereniging → de tuinverenigingen
• het tuinreglement → de tuinreglementen

Notice two things:

1. Dutch plural nouns usually take de, even if the singular had het.
2. The endings can vary:
   • -ing nouns often become -ingen
   • words like reglement often become -en

So:

• De tuinverenigingen hebben tuinreglementen.
  = The garden associations have garden regulations.

Because Dutch often expresses relationships by making a compound noun instead of using a phrase with van.

So Dutch prefers:

• tuinvereniging
• tuinreglement

rather than something like:

• vereniging van tuinen
• reglement van de tuin

Those longer structures are possible in some contexts, but the compound is the normal and natural choice here.

A rough pronunciation guide is:

• De ≈ duh
• tuinvereniging ≈ town-fuh-ray-nuh-khing
• heeft ≈ hayft
• een is often pronounced like a very weak uhn or even just n
• tuinreglement ≈ town-reh-gluh-ment

A few useful notes:

• ui in tuin is a difficult Dutch vowel that does not exist exactly in English.
• g in many Dutch accents is a throaty sound, especially in vereniging if pronounced carefully.
• In normal speech, een is often unstressed.

Yes, but it changes the nuance.

• De tuinvereniging heeft een tuinreglement.
  = The garden association has a set of regulations / one rulebook.

• De tuinvereniging heeft tuinreglementen.
  = The garden association has regulations / multiple regulations.

The original sentence presents tuinreglement as a single countable thing, like one official document or one formal set of rules.