Minä jään puistoon, koska aurinko paistaa.

Yes. In Finnish, the subject pronoun is often omitted when the verb ending already shows who the subject is.

• jään = I stay / I will stay
• the ending -n already tells you it is I

So:

• Minä jään puistoon, koska aurinko paistaa.
• Jään puistoon, koska aurinko paistaa.

Both are correct.

Including minä can add emphasis or clarity, a bit like I’m the one staying in the park.

jään is the 1st person singular form of the verb jäädä.

So:

• jäädä = to stay, remain, be left
• jään = I stay / I will stay / I am staying depending on context

Some forms of jäädä:

• minä jään = I stay
• sinä jäät = you stay
• hän jää = he/she stays
• me jäämme = we stay
• te jäätte = you (plural) stay
• he jäävät = they stay

Notice that jäädä is not a completely regular-looking verb, so learners often need to memorize its forms.

This is one of the most common questions.

• puistossa = in the park
• puistoon = into the park / to the park

But with the verb jäädä, Finnish often uses the illative form (-Vn, here -oon) to express the place where someone stays behind / remains / decides to stay.

So jään puistoon is the natural Finnish way to say something like:

• I’ll stay in the park
• I’m staying at the park
• I’ll remain in the park

Even though English uses in, Finnish often uses this destination-like form after jäädä.

Compare:

• Olen puistossa. = I am in the park.
  (simple location)
• Jään puistoon. = I’ll stay in the park / I’ll remain at the park.
  (the place where I am staying behind)

So this is partly a case meaning issue and partly a verb pattern you learn with jäädä.

If you translate word-for-word, it may look a bit like that, but that is not how you should understand it.

The ending -oon is the illative case, which often has the basic idea of into or to, but Finnish uses cases more idiomatically than English uses prepositions.

With jäädä, the illative often marks the place where someone remains.

So do not think:

• jään puistoon = bad English-style I stay into the park

Instead think:

• jään puistoon = natural Finnish expression meaning I’ll stay in the park

In this sentence, koska means because.

• Minä jään puistoon, koska aurinko paistaa.
• I’m staying in the park, because the sun is shining.

That is the most important meaning here.

Learners should know that koska can also mean when in some contexts, but in this sentence the meaning is clearly because.

For example:

• Koska aurinko paistaa, jään puistoon. = Because the sun is shining, I’m staying in the park.

So here koska introduces a reason clause.

Finnish does not change word order in the same dramatic way that languages like German do in subordinate clauses.

So after koska, the clause stays quite normal:

• aurinko paistaa = the sun shines / the sun is shining

That is the standard subject + verb order.

So:

• koska aurinko paistaa = because the sun is shining

There is no need to move the verb to the end.

Finnish uses the present tense more broadly than English does.

• aurinko paistaa literally looks like the sun shines
• but in context it often means the sun is shining

Finnish does not have a separate compulsory progressive form like English is shining, is staying, is raining.

So the present tense can cover both:

• general present: the sun shines
• current ongoing action: the sun is shining

Here, the natural English translation is usually the sun is shining.

Finnish does not have articles like English a, an, the.

So:

• aurinko can mean sun or the sun, depending on context

In this sentence, English naturally uses the sun, but Finnish just says:

• aurinko paistaa

You have to understand definiteness from context.

This is very common in Finnish:

• Koira nukkuu. = The dog is sleeping / A dog is sleeping
• Aurinko paistaa. = The sun is shining

Yes, in normal written Finnish, a comma is used before this kind of subordinate clause introduced by koska.

So:

• Minä jään puistoon, koska aurinko paistaa.

That comma is standard punctuation.

English also often uses a comma before because in some situations, but Finnish punctuation has its own rules. In a learner sentence like this, keeping the comma is the right choice.

Yes. Finnish has no separate future tense in the way English does.

So jään can mean:

• I stay
• I am staying
• I will stay

The exact meaning depends on context.

In this sentence, the most natural interpretation is often:

• I’ll stay in the park, because the sun is shining

So Finnish uses the present form where English may prefer a future form.

Yes, but it means something a little different.

• Minä jään puistoon = I will stay in the park / I am staying in the park
  → this emphasizes the decision to remain there
• Minä olen puistossa = I am in the park
  → this only states location

So if you want to express I’m remaining there instead of leaving, jään puistoon is better.

If you only want to say where you are, olen puistossa is better.

Yes, it is understandable and basically natural. It sounds like someone is deciding to remain at the park because the weather is nice.

In very everyday speech, a Finnish speaker might often omit minä:

• Jään puistoon, koska aurinko paistaa.

That may sound a little more natural in conversation unless you want emphasis on I.