Minä menen sisään, koska ulkona on kylmä.

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

• menen = I go / I am going

So:

• Minä menen sisään = I’m going inside
• Menen sisään = also I’m going inside

Including minä can add a little emphasis or clarity. In everyday Finnish, leaving it out is very common.

Menen is the 1st person singular present tense form of the verb mennä (to go).

Conjugation:

• minä menen = I go
• sinä menet = you go
• hän menee = he/she goes
• me menemme = we go
• te menette = you (plural) go
• he menevät = they go

So in the sentence:

• Minä menen sisään = I go inside / I’m going inside

Notice that the stem changes a bit:

• dictionary form: mennä
• 1st person: menen

That kind of change is normal with many Finnish verbs.

Because sisään expresses movement into something, while sisällä expresses being inside.

Compare:

• Menen sisään = I go inside
• Olen sisällä = I am inside

This is a very important Finnish distinction:

• sisään = into / insideward / going in
• sisällä = inside / being in
• sisästä = from inside

So here, since the speaker is moving from outside to inside, sisään is the correct choice.

For a learner, the simplest and most useful way to think of sisään is as a directional word meaning “inside / in” used with motion.

It belongs to a common set of location-and-direction words in Finnish, such as:

• ulos = out
• ulkona = outside
• ulkoa = from outside

and

• sisään = in / into
• sisällä = inside
• sisältä = from inside

You do not need to analyze it too deeply at first. What matters most is learning the pattern:

• motion to a place → words like sisään
• location in a place → words like sisällä
• motion from a place → words like sisältä

Ulkona means outside in the sense of being outside.

Examples:

• Olen ulkona. = I am outside.
• Lapsi leikkii ulkona. = The child is playing outside.

It contrasts with:

• ulos = out / to the outside
• ulkona = outside
• ulkoa = from outside

In your sentence:

• ulkona on kylmä = it is cold outside

So ulkona tells you the location where it is cold.

Because Finnish usually does not use a dummy subject like English it in weather or general-condition expressions.

English says:

• It is cold outside

Finnish says:

• Ulkona on kylmä

Literally, this is more like:

• Outside is cold

But in natural English, of course, we translate it as It is cold outside.

This is very normal in Finnish:

• On kylmä. = It’s cold.
• On myöhä. = It’s late.
• On pimeää. = It’s dark.

So do not look for an equivalent of English it here.

In on kylmä, kylmä is an adjective meaning cold.

• on kylmä = it is cold

This is the normal way to describe temperature or a general condition.

You may also see forms like on kylmää, but that is a different structure and is used in different contexts. For a beginner, the safest pattern to learn is:

• on kylmä = it is cold
• on kuuma = it is hot
• on hiljaista is another common pattern, but that involves a different kind of expression

So for this sentence, kylmä is exactly what you want.

Koska means because in this sentence. It introduces the reason.

Structure:

• Minä menen sisään = main clause
• koska ulkona on kylmä = reason clause

So:

• I’m going inside because it’s cold outside.

Be careful: koska can also mean when in some contexts, especially in older or more formal language, but for learners it is most often encountered as because.

Finnish normally uses a comma to separate a main clause from a subordinate clause.

So:

• Minä menen sisään, koska ulkona on kylmä.

This is standard punctuation.

The part after koska is a subordinate clause, so the comma is expected in standard written Finnish.

This word order is the most neutral and natural one:

• Minä menen sisään, koska ulkona on kylmä.

Finnish word order is more flexible than English, but changes in order often change the emphasis.

For example, you could also say:

• Koska ulkona on kylmä, minä menen sisään.

This puts more focus on the reason first:

• Because it’s cold outside, I’m going inside.

Both are grammatical. The original sentence is just the most straightforward everyday order.

Sometimes yes, but they are not always exactly the same.

• sisään = inside, into the interior
• sisälle = onto/into the inside area of something, often with a slightly different spatial feel

In many everyday situations, especially with buildings, both may be heard:

• Menen sisään
• Menen sisälle

Both can mean I’m going inside.

However, sisään is very common when you simply mean entering indoors. For a learner, mennä sisään is an excellent basic pattern to remember.

It is present tense, but Finnish present tense often covers what English expresses with either present or near future.

So:

• Minä menen sisään can mean
  • I go inside
  • I am going inside
  • I’ll go inside (in the right context)

Finnish does not have a separate future tense in the way English does. The meaning comes from context.

In this sentence, the natural English translation is usually:

• I’m going inside, because it’s cold outside.

In everyday spoken Finnish, a person would very often omit minä:

• Menen sisään, koska ulkona on kylmä.

Depending on the speaker and dialect, spoken forms can become even more reduced, but for standard Finnish, this version is already very natural.

So a useful rule is:

• written/neutral standard: Minä menen sisään...
• common natural speech: Menen sisään...

A very useful thing to learn is the location-direction pattern:

Inside / outside

• sisään = into, inside
• sisällä = inside

• sisältä = from inside

• ulos = out
• ulkona = outside
• ulkoa = from outside

And also a few common verbs and expressions:

• mennä = to go
• menen = I go
• on kylmä = it is cold
• koska = because

This kind of pattern-building is especially helpful in Finnish, because many words work in matching directional sets.