Sivuovi avautui hitaasti, koska lukko oli vanha.

Finnish often makes compound nouns by joining two nouns together.

• sivu = side
• ovi = door

So sivuovi literally means side door.

This is very normal in Finnish. English often writes these as two words, but Finnish usually combines them into one.

Avautui is the past tense of avautua, which means to open or to open up in an intransitive/reflexive sense.

• avautua = to open (by itself / become open)
• avata = to open (something)

So:

• Ovi avautui. = The door opened.
• Hän avasi oven. = He/she opened the door.

In your sentence, the side door is the thing that opened, so avautui is the natural choice.

It marks a past tense form here.

The verb type is a little tricky because the basic verb is avautua. In the past tense, it becomes:

• avautua → avautui

So avautui means opened or was opening/opened up depending on context.

You do not need to separate -ui as a single universal past ending for every verb, but it is part of the way this particular verb forms the past tense.

Because hitaasti is an adverb, and hidas is an adjective.

• hidas = slow
• hitaasti = slowly

In the sentence, the word describes how the door opened, so Finnish uses the adverb:

• Sivuovi avautui hitaasti. = The side door opened slowly.

A very common Finnish pattern is:

• adjective stem + -sti → adverb

For example:

• nopea = fast
• nopeasti = quickly

Not always. Koska can mean:

1. because
2. when

In this sentence, it clearly means because:

• Sivuovi avautui hitaasti, koska lukko oli vanha.
• The side door opened slowly, because the lock was old.

Context tells you which meaning is intended.

Examples:

• Koska olin väsynyt, menin nukkumaan. = Because I was tired, I went to sleep.
• Koska tulit kotiin? = When did you come home?

So learners need to pay attention to context.

This is a very basic Finnish sentence structure:

• lukko = the lock
• oli = was
• vanha = old

So it is simply The lock was old.

Finnish often uses straightforward noun + olla + adjective structure:

• Talo oli suuri. = The house was big.
• Kahvi on kuumaa. = The coffee is hot.

Nothing special is happening here grammatically.

Finnish has no articles, so there is no direct equivalent of a/an or the.

That means:

• sivuovi can mean a side door or the side door
• lukko can mean a lock or the lock

The exact meaning depends on context. In your sentence, English naturally uses the side door and the lock, but Finnish does not need separate article words.

This is one of the biggest differences from English.

They are both subjects of their clauses, so the basic nominative form is used.

Main clause:

• Sivuovi avautui hitaasti.
• sivuovi is the subject

Reason clause:

• koska lukko oli vanha
• lukko is the subject

In Finnish, subjects are often in the nominative singular when talking about one thing in a normal sentence.

The sentence uses a very common neutral word order:

• Sivuovi = subject
• avautui = verb
• hitaasti = adverb
• koska lukko oli vanha = reason clause

So:

Subject + verb + adverb + because-clause

Finnish word order is more flexible than English, but this version sounds natural and neutral.

You could move parts around for emphasis, but the original is the most straightforward form for learners.

Yes, a few useful things:

• sivuovi has a long vowel sequence in the middle: ...uo...
• avautui also has several vowels together, which is normal in Finnish
• hitaasti has double aa, and long vowels matter in Finnish pronunciation
• stress in Finnish is usually on the first syllable

Roughly:

• SI-vu-o-vi
• A-vau-tui
• HI-taa-sti
• KOS-ka
• LUK-ko
• O-li
• VAN-ha

The most important point is that vowel length and consonant length can change meaning in Finnish, so pronounce long sounds clearly.

Yes, that is one natural implication of avautui.

Because avautua is the intransitive/reflexive-style verb, it focuses on the door becoming open, not on a person opening it.

So the sentence suggests:

• the side door opened
• it happened slowly
• the reason was that the lock was old

It does not explicitly say who or what caused it. The focus is on the event, not the agent.

Because it is a predicate adjective after oli.

In Finnish, after olla (to be), an adjective often stays in the basic nominative singular when describing a singular subject:

• Lukko oli vanha. = The lock was old.
• Ovi oli raskas. = The door was heavy.

So vanha is just the normal singular form agreeing with lukko in a basic descriptive sentence.