Learn Hardcore Dutch: Door een vertraging missen zij hun overstap en komen ze te laat op het werk. - Because of a delay they miss their transfer and arrive at work too late.

door een vertraging = because of a delay / due to a delay
Spatial door (through) would be like: Door de deur lopen = to walk through the door.

So the structure is:
Door een vertraging (because of a delay) missen zij hun overstap (they miss their connection)...

Why is the word order missen zij hun overstap instead of zij missen hun overstap?

Dutch main clauses follow the V2 rule: the finite verb (here: missen) must be in second position in the clause.

The sentence begins with an adverbial phrase:

Door een vertraging (because of a delay) – this takes the first position.

So the finite verb must come next:

Door een vertraging – position 1
missen – position 2 (finite verb)
zij hun overstap – the rest of the sentence

If you start with the subject instead, you get:

Zij missen hun overstap door een vertraging.

Both are correct. The original just chooses to emphasize the cause by placing Door een vertraging first.

Why do we see both zij and ze for they in the same sentence?

Dutch has two forms of they:

zij – the stressed form (used for emphasis or when introducing the subject clearly)
ze – the unstressed / weak form (more neutral in the flow of the sentence)

In the sentence:

missen zij hun overstap – zij can sound a bit more prominent: they miss their connection.
en komen ze te laat op het werk – ze here is unstressed and flows more naturally.

In everyday speech and writing, many people would also say:

Door een vertraging missen ze hun overstap en komen ze te laat op het werk.

So using zij in the first clause is stylistic and slightly more emphatic, not a grammatical necessity.

What exactly does overstap mean here?

Overstap literally means transfer / change (of trains, buses, etc.).

In this context:

hun overstap missen = to miss their connection / transfer (e.g. between trains)

Related words:

overstappen = to change / transfer (verb)
- Ik moet in Utrecht overstappen. = I have to change (trains) in Utrecht.
aansluiting can also be used for a connecting train/bus:
- Mijn aansluiting missen = to miss my connection

So overstap is the act or instance of changing from one line/vehicle to another.

Why is it hun overstap and not hen overstap? What’s the difference between hun and hen?

Here hun is a possessive pronoun: their.

hun overstap = their transfer / their connection

The basic distinctions:

hun = their (possessive)
- hun huis (their house)
- hun overstap (their transfer)
hen = them (as a direct or indirect object, mainly after prepositions or in more formal style)
- Ik zie hen. (I see them.)
- Met hen praten (to talk with them)

In everyday spoken Dutch, many people overuse hun and even say things like Ik zie hun (non‑standard), but in standard grammar:

Use hun for possession.
Use hen for them as an object in more careful/formal Dutch.

In this sentence, only the possessive hun makes sense.

What does te laat literally mean, and is te always “too”?

Literally:

te = too
laat = late

So te laat = too late.

In practice, te laat is also used where English might just say late:

Ik ben te laat. = I’m late / I’m too late (context decides how strong it feels).

Yes, te normally means too (much):

te warm = too warm
te duur = too expensive
te snel = too fast

But in te laat, it’s such a fixed phrase that it often just means “late” in everyday usage, without necessarily being strongly emphatic.

Why is it op het werk and not in het werk or just op werk?

Op het werk is a set expression meaning at work (at one’s workplace).

op het werk komen = to arrive at work
te laat op het werk komen = to arrive late at work

Differences:

op het werk – at the workplace (where you work; office, factory, etc.)
in het werk – literally in the work, used for things like:
- in het werk dat hij doet = in the work that he does
op werk – without the article, sounds incomplete/odd here in standard Dutch; you generally say op het werk for “at work”.

So the natural idiomatic phrase is op het werk.

Could you also say Door een vertraging missen ze hun overstap en komen ze te laat op het werk? Is that more natural?

Yes, that version is perfectly correct and very natural:

Door een vertraging missen ze hun overstap en komen ze te laat op het werk.

Using ze instead of zij makes the sentence sound a bit more neutral and informal. The meaning is the same; the original with zij just gives slightly more weight/emphasis to they in the first clause.

So both are fine:

missen zij hun overstap – slightly more emphatic, somewhat more formal/written
missen ze hun overstap – neutral, very common in speech

Why is there no comma before en even though there are two clauses?

Dutch comma rules are generally looser than English ones. With two main clauses joined by en, a comma is often optional.

In this sentence:

missen zij hun overstap – main clause 1
(en) komen ze te laat op het werk – main clause 2

You may write:

Door een vertraging missen zij hun overstap, en komen ze te laat op het werk.

but in modern Dutch it’s very common (and stylistically preferred by many) to leave the comma out when the clauses are short and closely connected:

Door een vertraging missen zij hun overstap en komen ze te laat op het werk.

So the version without the comma is standard and natural.

Can door een vertraging be replaced by vanwege een vertraging or door de vertraging? Are there differences?

All of these are possible, with small nuances:

Door een vertraging
- because of a delay (non‑specific; some delay happened)
Door de vertraging
- because of the delay (a specific delay that is already known in the context)
- e.g. one particular train’s delay you’ve just been talking about.
Vanwege een vertraging
- also because of a delay, slightly more formal/neutral than door for some speakers.
- Often used in announcements:
  - Vanwege een vertraging vertrekt de trein later.

In everyday conversation, door een vertraging is very natural and common.