Omdat het stoffig is, moet ik ineens niesen zodra ik de schuur binnenloop.

Because omdat introduces a subordinate clause, and in Dutch subordinate clauses the conjugated verb usually goes to the end.

So:

• het is stoffig = it is dusty
• omdat het stoffig is = because it is dusty

This is a very common pattern in Dutch:

• omdat ik moe ben = because I am tired
• omdat hij geen tijd heeft = because he has no time

So the final is is there because of the clause introduced by omdat.

That is because the sentence begins with the subordinate clause Omdat het stoffig is. After a clause like that, Dutch uses inversion in the main clause: the verb comes before the subject.

Normal main clause order:

• Ik moet ineens niesen.

But after the opening clause:

• Omdat het stoffig is, moet ik ineens niesen.

This is a standard Dutch rule:

• Als het regent, blijf ik thuis.
• Wanneer hij komt, ga ik weg.

So moet ik is not special to this sentence; it is regular word order after an introductory clause.

Omdat means because and introduces a subordinate clause.

• Omdat het stoffig is, moet ik ineens niesen.

Want also means because, but it introduces a main clause, so the word order stays normal:

• Ik moet ineens niesen, want het is stoffig.

Compare:

• omdat het stoffig is → verb at the end
• want het is stoffig → normal main-clause order

In many everyday contexts, both are possible, but the grammar after them is different.

Here moet comes from moeten, which can mean must, have to, or sometimes something like can’t help depending on context.

In this sentence, it does not sound like an obligation, as if someone is forcing you to sneeze. It expresses something more like:

• I have to sneeze
• I suddenly need to sneeze
• I can’t help sneezing

That use is very natural in Dutch with bodily reactions:

• Ik moet hoesten. = I have to cough.
• Ik moet gapen. = I have to yawn.
• Ik moet niezen. = I have to sneeze.

After modal verbs like moeten, Dutch uses the bare infinitive, without te.

So:

• ik moet niesen
• ik kan komen
• ik wil gaan

Not:

• ik moet te niesen

This is the same idea as English I must sneeze rather than I must to sneeze.

Common Dutch modal verbs that work this way include:

• kunnen = can
• moeten = must / have to
• mogen = may / be allowed to
• willen = want to
• zullen = shall / will

Ineens means suddenly or all of a sudden.

In this sentence it adds the idea that the urge to sneeze comes on abruptly:

• moet ik ineens niesen = I suddenly have to sneeze

It is common in spoken and written Dutch. A close synonym is plotseling.

Compare:

• Ik moest ineens lachen. = I suddenly had to laugh.
• Het begon ineens te regenen. = It suddenly started raining.

Zodra means as soon as.

It introduces another subordinate clause, so the verb goes to the end again:

• zodra ik de schuur binnenloop = as soon as I walk into the shed

Just like with omdat, the conjugated verb is at the end of that clause.

Other examples:

• Zodra hij thuiskomt, eten we. = As soon as he gets home, we eat.
• Zodra ik tijd heb, bel ik je. = As soon as I have time, I’ll call you.

So in this sentence there are two subordinate clauses:

• Omdat het stoffig is
• zodra ik de schuur binnenloop

Yes. Binnenlopen is a separable verb.

The full infinitive is:

• binnenlopen = to walk in / enter

In a normal main clause, separable verbs split:

• Ik loop de schuur binnen.

But in a subordinate clause, the parts usually stay together at the end:

• zodra ik de schuur binnenloop

So compare:

• main clause: ik loop binnen
• subordinate clause: dat ik binnenloop

This is a very important Dutch pattern.

Other examples:

• opstaan → Ik sta op / omdat ik vroeg opsta
• meebrengen → Ik breng het mee / als ik het meebreng

Yes, in some contexts you may hear or see inlopen or de schuur in lopen, but binnenlopen is a very natural choice for walk into / enter.

The version in your sentence, de schuur binnenlopen, is especially common and idiomatic for physically entering a space.

Dutch often allows a few similar ways to express movement into somewhere:

• de schuur binnenlopen
• de schuur in lopen
• de schuur ingaan

They are close in meaning, but binnenlopen sounds very natural here.

Because schuur is a de-word in Dutch.

So:

• de schuur = the shed
• een schuur = a shed

Unfortunately, noun gender/article choice often just has to be learned with the noun. English speakers usually need to memorize whether a noun takes de or het.

A useful habit is to learn nouns together with their article:

• de schuur
• het huis
• de tafel

Yes, stoffig means dusty.

It is related to stof, which means dust. The suffix -ig often turns a noun into an adjective meaning full of or characterized by that thing.

So:

• stof = dust
• stoffig = dusty

You will see similar patterns in Dutch:

• modder = mud → modderig = muddy
• zout = salt → zoutig = salty
• grap = joke → grappig = funny

The spelling stoffig with double f is just regular Dutch spelling after adding the suffix.

Yes. A very common alternative is:

• Ik moet ineens niesen zodra ik de schuur binnenloop, omdat het stoffig is.

Or more simply:

• Ik moet ineens niesen, omdat het stoffig is.

Dutch is flexible about where subordinate clauses go. The version with Omdat het stoffig is at the beginning puts the reason first, which may sound a bit more explanatory or slightly more formal in tone.

So both are correct:

• Omdat het stoffig is, moet ik ineens niesen zodra ik de schuur binnenloop.
• Ik moet ineens niesen zodra ik de schuur binnenloop, omdat het stoffig is.

Because that opening subordinate clause is followed by the main clause.

Dutch normally uses a comma between a subordinate clause and the main clause when the subordinate clause comes first:

• Omdat het regent, blijf ik thuis.
• Als hij belt, zeg ik het wel.

So in your sentence:

• Omdat het stoffig is, moet ik ineens niesen zodra ik de schuur binnenloop.

The comma helps mark the transition from the subordinate clause to the main clause.