Learn Hardcore Finnish: Pääsen sisään vasta, kun opas avaa sivuoven. - I can get inside only when the guide opens the side door.

Finnish present tense often covers both “I do” and “I can / I get to” depending on the verb and context.
päästä means “to get (to), to be able to access, to gain entry”. So pääsen sisään is a very common idiom meaning “I get in / I can get in / I’m able to enter”.

What is the dictionary form of pääsen, and why does it change to pääsen?

Dictionary form: päästä (“to get in/to reach/to be allowed to”).
Conjugation in present tense:

minä pääsen (I get in / I can get in)
sinä pääset
hän pääsee, etc.

The stem changes because Finnish verbs inflect and also show consonant gradation patterns; here you get the present stem pääse- + -n for 1st person singular.

What exactly is sisään grammatically? Is it a case ending?

sisään is an adverb meaning “inside / in(to)”. Historically it relates to the illative (“into”), but in modern Finnish it’s treated as a fixed adverb in phrases like:

mennä sisään = “to go in”
päästä sisään = “to get in”

You don’t need to attach it to a noun here; it stands alone.

Why isn’t there a word for “I” (minä) in the sentence?

Finnish usually drops subject pronouns because the verb ending already shows the person.
So pääsen already means “I get in / I can get in”, and minä would only be added for emphasis or contrast.

What does vasta add here? Does it mean “only”?

Yes—vasta often means “only” in the sense of “not until” / “only when” / “only after”.
In this sentence it signals a condition/time limit: you do not get in earlier; you get in only once the next thing happens.

Common patterns:

vasta kun ... = “only when ...”
vasta sitten = “only then”
vasta myöhemmin = “only later”

Why is there a comma before kun?

Because kun opas avaa sivuoven is a subordinate clause (“when the guide opens the side door”). Finnish typically separates subordinate clauses with a comma, especially when the subordinate clause comes after the main clause.

Does kun mean “when” or “because” here? I’ve seen both.

kun can mean both “when” and “because”, but here it’s clearly “when” because it expresses the time/condition for getting in: only when the guide opens the side door.
A “because” reading would usually explain a reason rather than a timing condition, and it would sound different in context.

Why is it opas and not some case like oppaan?

opas is the subject of the subordinate clause, so it’s in the nominative case:
opas avaa ... = “the guide opens ...”

You’d get oppaan in other roles, e.g. genitive (“the guide’s”) or certain object constructions, but not as a normal subject.

Why is the object sivuoven and not sivuovi or sivuovea?

Finnish marks objects with different cases depending on whether the action is seen as complete/whole vs. ongoing/partial.

avaa sivuoven uses the total object (often called genitive/accusative in descriptions). It treats the opening as a complete event: “opens the side door (i.e., opens it up)”.
avaa sivuovea (partitive) would suggest something less bounded/ongoing, like “is opening the side door” (process-focused), or it may imply repeated/indefinite action depending on context.

In a one-time event that enables entry, sivuoven is the natural choice.

What is the base form of sivuoven, and what’s happening to the ending?

Base form: sivuovi (“side door”).
It inflects like:

nominative: sivuovi
genitive: sivuoven
partitive: sivuovea

The stem changes to sivuove- in many forms, and the genitive ending is -n, giving sivuoven.

Could the word order be different, like Kun opas avaa sivuoven, pääsen sisään vasta?

Yes, that’s also correct. If the kun-clause comes first, you still use a comma:

Kun opas avaa sivuoven, pääsen sisään vasta.

It feels slightly more “setup first, result second”. With vasta, many speakers still prefer keeping vasta close to what it modifies (the “only then” idea), but both versions are understandable and grammatical.