Viime viikolla hän ei käynyt teatterissa, koska hänellä ei ollut aikaa.

Because Finnish often uses a case ending in time expressions.

Here, viikolla is the adessive form of viikko (week), and viime viikolla means last week.

So:

• viikko = week
• viikolla = during/on/in a week, depending on context
• viime viikolla = last week

This is just the normal Finnish way to express this idea.

Finnish does not have articles like English a/an and the.

So teatterissa can mean:

• in the theatre
• at the theatre

The context tells you what sounds natural in English.

That is why Finnish sentences often look shorter than English ones.

Finnish negation uses a special negative verb, ei, which is conjugated for the subject.

For hän:

• hän ei = he/she does not, he/she did not

In the past tense, the main verb appears in a special form:

• hän kävi = he/she went / visited
• hän ei käynyt = he/she did not go / did not visit

So the negative comes before the main verb, not after it.

Yes, it is the same basic verb form, but the tense is different because of the helper around it.

Compare:

• hän kävi = he/she went
• hän ei käynyt = he/she did not go
• hän on käynyt = he/she has gone / has visited
• hän ei ole käynyt = he/she has not gone / has not visited

So käynyt appears in both:

• the negative past
• the perfect tense

You tell them apart from the other verb:

• ei alone -> negative past
• on / ei ole -> perfect

Because with the verb käydä, Finnish normally uses the place in a form that looks like in/at that place.

So:

• käydä teatterissa = to go to the theatre / to visit the theatre
• literally something like to visit at the theatre

This is just how käydä works.

By contrast:

• mennä teatteriin = to go to the theatre

So both can translate similarly in English, but the Finnish verb changes the case:

• käydä + teatterissa
• mennä + teatteriin

They are close in meaning, but not identical.

• mennä teatteriin focuses on the movement: to go to the theatre
• käydä teatterissa means to go/visit the theatre as an event or outing

Very roughly:

• mennä = go
• käydä = go and be there / visit

In this sentence, käydä teatterissa sounds very natural because it is about whether the person went to the theatre as an activity last week.

That is exactly the structure Finnish uses for possession and availability.

Finnish often says that something is on someone rather than using have.

So:

• hänellä on aikaa = he/she has time
• hänellä ei ole aikaa = he/she does not have time
• hänellä ei ollut aikaa = he/she did not have time

Here:

• hänellä = on him/her
• oli / ollut = was
• aikaa = time

This is one of the most important Finnish patterns to learn:

• minulla on... = I have...
• sinulla on... = you have...
• hänellä on... = he/she has...

Because aikaa is the partitive form of aika, and that is the normal choice here.

There are two big reasons:

1. Negation
   Finnish often uses the partitive in negative sentences.

2. Uncountable / indefinite amount
   Time here is not one complete, countable item. It means enough time / any time in a general sense.

So:

• aika = time
• aikaa = (some/any) time

That is why:

• hänellä ei ollut aikaa = he/she did not have time

Because Finnish negative past tense is built with:

• the negative verb ei
• plus a special form of the main verb

Different verbs have different past negative forms:

• käydä -> käynyt
• olla -> ollut

So:

• hän ei käynyt = he/she did not go
• hänellä ei ollut aikaa = he/she did not have time

The pattern is the same, even though the verb forms look different.

It can mean either.

Finnish hän is gender-neutral:

• he
• she

You only know which one is meant from context.

That is why the same Finnish sentence could refer to a man or a woman.

Finnish word order is flexible, and the beginning of the sentence often shows what information is being framed first.

Starting with Viime viikolla sets the time first:

• Last week, ...

This is very natural when the speaker wants to locate the whole situation in time before saying what happened.

A more neutral English-like order is possible too:

• Hän ei käynyt teatterissa viime viikolla, koska hänellä ei ollut aikaa.

But Viime viikolla first is very normal Finnish.

Because koska introduces a subordinate clause:

• koska hänellä ei ollut aikaa = because he/she did not have time

In standard Finnish spelling, such clauses are separated with a comma.

So the comma is there for the same basic reason as in English.

Yes.

You could also say:

• Koska hänellä ei ollut aikaa, hän ei käynyt teatterissa viime viikolla.

That means the same thing, but the emphasis changes a little:

• the original starts with the time
• this version starts with the reason

Both are grammatical.

In this sentence, koska means because.

That is the most common meaning in modern Finnish.

In some contexts, especially older or more literary language, koska can also mean when, but learners should usually read it as because unless the context clearly shows otherwise.

Here there is no ambiguity:

• koska hänellä ei ollut aikaa = because he/she did not have time

Because literal case meanings and natural English translations are not always the same.

With käydä, Finnish uses the location form:

• teatterissa = in/at the theatre

But idiomatic English usually says:

• go to the theatre

So the Finnish grammar and the English translation do not line up word for word.

That is very common in Finnish, and it is better to learn whole patterns like:

• käydä koulussa = go to school
• käydä lääkärissä = go to the doctor
• käydä teatterissa = go to the theatre