In onze woonwijk heeft bijna elk huis een brievenbus naast de deur.

Because Dutch main clauses follow the verb-second (V2) rule. The first position is taken by In onze woonwijk, so the finite verb heeft must come next.

Basic order:

• Bijna elk huis heeft een brievenbus naast de deur.

With the location moved to the front:

• In onze woonwijk heeft bijna elk huis een brievenbus naast de deur.

So this is not a question order; it is normal Dutch word order after fronting an element.

The subject is bijna elk huis, and the key word there is huis, which is singular.

• elk huis = every house
• bijna elk huis = almost every house

Even though the meaning refers to many houses, grammatically it is still singular, so Dutch uses the singular verb:

• elk huis heeft
• not elk huis hebben

Because bijna modifies the whole idea elk huis.

• bijna elk huis = almost every house

Dutch usually places bijna directly before the word or phrase it modifies. So:

• bijna elk huis is correct
• elk bijna huis is not correct

A common alternative is:

• bijna alle huizen = almost all houses

Because huis is a het-word and singular.

With elk / elke:

• elk
  • singular het-word → elk huis
• elke
  • singular de-word → elke straat

So:

• elk huis
• elke deur
• elke woonwijk

Also, elk is only used with singular nouns.

They are very similar and often both mean every or each.

In this sentence, you could also say:

• In onze woonwijk heeft bijna ieder huis een brievenbus naast de deur.

Very roughly:

• elk often feels a bit more like each/every single
• ieder / iedere can sound slightly more formal or neutral

In everyday use, the difference is usually small.

Forms:

• elk huis
• ieder huis
• elke straat
• iedere straat

Because woonwijk is a de-word: de woonwijk.

With our, Dutch uses:

• ons for singular het-words
• onze for de-words and plurals

So:

• ons huis because huis is a het-word
• onze woonwijk because woonwijk is a de-word
• onze huizen because plurals take onze

Because Dutch writes compound nouns as one word.

• brief = letter
• bus = box/container
• brievenbus = mailbox / letterbox

This is very normal in Dutch:

• treinstation
• huisdeur
• woonwijk

English often uses separate words where Dutch uses a compound.

Because the sentence is talking about a mailbox as a general feature of each house, not one specific mailbox already known in the conversation.

• een brievenbus = a mailbox
• de brievenbus = the mailbox

Here the idea is: each house has a mailbox. So the indefinite article een is the natural choice.

Because the door is understood as a specific, identifiable door belonging to the house, usually the main/front door.

So:

• naast de deur = next to the door

Using een deur would sound more like next to some door, which is less natural here.

Dutch often uses the definite article when the thing is obvious from the context.

It describes the location of de brievenbus.

So the structure is:

• een brievenbus naast de deur = a mailbox next to the door

It tells you where the mailbox is. In this sentence, it does not describe the house or the neighborhood.

Yes, absolutely. That is also correct and very natural.

Compare:

• In onze woonwijk heeft bijna elk huis een brievenbus naast de deur.
• Bijna elk huis in onze woonwijk heeft een brievenbus naast de deur.

The difference is mainly focus:

• Starting with In onze woonwijk emphasizes the location
• Starting with Bijna elk huis emphasizes the houses

Both are good Dutch.