Jos ovi on auki, suljen sen heti.

Jos means if and introduces a conditional subordinate clause. The structure is:

• Jos + clause, then the main clause. So Jos ovi on auki = If the door is open.

In Finnish, it’s standard to use a comma between a subordinate clause and the main clause when the subordinate clause comes first:

• Jos ... , ... If you reverse the order, you still usually use a comma:
• Suljen sen heti, jos ovi on auki.

The neutral, complete sentence uses the verb olla (on = is) to link the subject to its state:

• ovi on auki = the door is open You may see Ovi auki! in signs/instructions or casual speech, but the full form with on is the standard general statement.

Auki behaves like a predicative word meaning open (often treated as an adverb-like predicate complement in Finnish descriptions). It commonly appears in expressions like:

• on auki = is open
• on kiinni = is closed It doesn’t inflect like a typical adjective in this use; it’s more of a fixed state expression.

Suljen is the 1st person singular present form of sulkea (to close):

• (minä) suljen = I close / I will close The -n ending signals I, so the subject pronoun minä is usually unnecessary.

Sen is the object form of se (it/that). Here it functions as a definite total object (you close the door completely), so Finnish uses the accusative-type form that looks like genitive:

• suljen sen = I close it (completely)

Yes, that word order is normal. The meaning stays the same, but the emphasis changes:

• Jos ovi on auki, suljen sen heti. emphasizes the condition first.
• Suljen sen heti, jos ovi on auki. emphasizes the action first.

Niin can be used like “then” to highlight the consequence (“if X, then Y”). It’s common in speech and fine in writing, but it’s optional:

• Jos ..., (niin) ...