Avaan sivuoven varovaisemmin, ettei se häiritse ketään.

Avaan is the 1st person singular form of the verb avata (to open), in the present indicative:

• (minä) avaan = I open / I’m opening / I will open (Finnish present often covers near-future too, depending on context).

Sivuoven is the object in the so-called total object form, which often looks like genitive singular -n.

• Base word: sivuovi (side door)
• Total object (singular): sivuoven

It’s used because the action is seen as complete/whole (opening the door, as a complete event). If you wanted to emphasize an incomplete/ongoing action, you could use a partitive object:

• avaan sivuovea = opening the side door (not necessarily fully / focusing on the process)

Yes, sivuovi is a compound noun:

• sivu = side
• ovi = door
  So sivuovi = side door, and sivuoven is its inflected form.

Varovaisemmin is the comparative form of the adverb:

• varovasti = carefully
• varovaisemmin = more carefully

Using the comparative often implies a comparison to some baseline (e.g., “more carefully than usual / than before / than I otherwise would”).

A common pattern is:

• adjective/adverb base → comparative with -mpi/-mmi- and adverb ending -n

Here:

• varovainen (careful, adjective)
• comparative stem: varovaisempi
• adverb comparative: varovaisemmin

You’ll see the -mm- in many adverb comparatives (e.g., nopeammin = faster).

Ettei is essentially a merged form of että ei:

• että = that (also used to introduce subordinate clauses)
• ei = not

So ettei introduces a negative subordinate clause, very often a purpose or result-to-avoid idea: so that … not / so as not to ….

Finnish normally uses a comma before a subordinate clause:

• Main clause: Avaan sivuoven varovaisemmin
• Subordinate clause: ettei se häiritse ketään

So the comma marks the boundary between the main clause and the subordinate clause.

Because the subordinate clause is negative, introduced by ettei (= “that not / so that not”). In Finnish, a negative clause uses:

• a negative verb (ei, or here it’s embedded in ettei)
• and the main verb in the connegative form (a special form without personal ending)

So:

• positive: se häiritsee = it disturbs
• negative: se ei häiritse = it does not disturb
  And with ettei, the ei is “built in,” so you get:
• ettei se häiritse (not häiritsee)

Se literally means it. In this kind of sentence it commonly refers to the action/event (opening the door) or the door as the source of disturbance (noise, etc.). Finnish se can point quite flexibly to the relevant “it” from context.

Ketään is the partitive form of kuka used in negative contexts, meaning anyone / anybody (as an object).

• ketään is typical after a negative verb: ei häiritse ketään = doesn’t disturb anyone

Kukaan is the nominative form typically used as a subject with negation:

• kukaan ei häiritse minua = nobody disturbs me

Here, anyone is the object of häiritse, so ketään is the natural choice.

Yes, Finnish word order is somewhat flexible, mainly changing emphasis rather than the core meaning. For example:

• Avaan varovaisemmin sivuoven, ettei se häiritse ketään. (emphasizes more carefully)
• Sivuoven avaan varovaisemmin, ettei se häiritse ketään. (emphasizes the side door)

The original order is neutral and very natural.