En pääse bussiin, koska se on täynnä.

Because Finnish often leaves out the subject pronoun when the verb already shows who the subject is.

In En pääse bussiin:

• en already tells you it means I do not
• so minä is not necessary

You can say Minä en pääse bussiin, but that sounds more emphatic, like:

• I can’t get on the bus
• or I, at least, can’t get on

The version without minä is the most neutral and natural.

Finnish forms negation with a special negative verb. That is why en is not just a separate word like English not.

In en pääse:

• en = the 1st person singular negative verb, meaning I do not
• pääse = the main verb in its special negative form

The dictionary form is päästä.

A few forms:

• en pääse = I can’t / I don’t get
• et pääse = you can’t
• ei pääse = he/she/it can’t
• emme pääse = we can’t
• ette pääse = you all can’t
• eivät pääse = they can’t

So the person is marked on en, not on the main verb in the usual way.

Here päästä means something like:

• to get in
• to get onto
• to gain access
• to manage to get somewhere

So En pääse bussiin is not just a general I can’t. It specifically suggests that something prevents you from getting onto the bus.

That is why it fits well with koska se on täynnä. The bus is full, so you cannot get on it.

Because Finnish uses case endings to show direction and location.

bussiin is the illative form of bussi, and it means:

• into the bus
• idiomatically in English: onto the bus / on the bus

So:

• bussi = bus
• bussiin = into the bus / onto the bus
• bussissa = in the bus / on the bus
• bussista = out of the bus

In this sentence, the speaker is talking about getting onto the bus, so a movement-toward form is needed: bussiin.

That is just a difference in how the two languages express the idea.

English says:

• on the bus
• on the train
• on the plane

Finnish usually treats vehicles as spaces you go into, so it uses the internal location cases:

• bussiin = onto / into the bus
• bussissa = on / in the bus
• bussista = off / out of the bus

So even though English uses on, Finnish uses the same case system it would use for going into an enclosed space.

Koska means because and introduces the reason.

So the structure is:

• En pääse bussiin = I can’t get on the bus
• koska se on täynnä = because it is full

This is the most straightforward and common way to give a reason in a sentence like this.

There are other ways to express because in Finnish, but koska is the basic one learners should know first.

Se refers to the bus.

In standard Finnish:

• se = it / that
• hän = he / she

Because bussi is a thing, the correct pronoun is se.

So:

• se on täynnä = it is full

A useful extra note: in colloquial Finnish, se is often also used for people, but in this sentence it simply means it.

Because Finnish normally uses täynnä in the expression to be full.

So:

• bussi on täynnä = the bus is full

By contrast, täysi is often used more like an ordinary adjective before a noun, for example:

• täysi bussi = a full bus

So the most natural pattern is:

• täysi + noun
• noun + on täynnä

Examples:

• täysi lasi = a full glass
• Lasi on täynnä. = The glass is full.

Yes, Finnish word order is fairly flexible, although some orders sound more neutral than others.

The given sentence:

• En pääse bussiin, koska se on täynnä.

is the most neutral order.

You could also say:

• Koska se on täynnä, en pääse bussiin.

This puts the reason first:

• Because it’s full, I can’t get on the bus.

You might also hear something like:

• Bussiin en pääse, koska se on täynnä.

This gives extra emphasis to bussiin, as if contrasting it with something else.

So yes, the order can change, but the original version is the safest default for learners.

Both can be possible, but they are not exactly the same.

• En pääse bussiin = I can’t get onto the bus / I’m unable to get on
• En voi mennä bussiin = I can’t go onto the bus

päästä often suggests that access is blocked by some circumstance.
That makes it especially natural here, because the bus is full.

So this sentence feels very idiomatic:

• En pääse bussiin, koska se on täynnä.

It sounds like there is no room, so getting on is impossible.

It is made by putting the noun into the illative singular, the case that often means into.

Starting from:

• bussi

you get:

• bussiin

With many words, the illative singular has a long vowel plus -n.
Here that appears as -iin.

So learners often notice:

• bussi → bussiin
• talo → taloon
• juna → junaan

The exact spelling pattern depends on the type of word, but the main idea is the same: this case marks movement into / onto something.