Ovaj autobus je spor, pa uvek stiže kasno.

Ovaj means this.

So ovaj autobus = this bus.

It is a demonstrative word, used to point out a specific thing that is close, known, or being referred to right now.

• ovaj autobus = this bus
• taj autobus = that bus
• onaj autobus = that bus over there / that one

In this sentence, ovaj tells us we are talking about a particular bus, not buses in general.

Because ovaj has to agree with autobus in gender, number, and case.

Here, autobus is:

• masculine
• singular
• nominative (because it is the subject of the sentence)

So the correct form is:

• masculine singular nominative: ovaj
• feminine singular nominative: ova
• neuter singular nominative: ovo

Examples:

• ovaj autobus = this bus
• ova knjiga = this book
• ovo selo = this village

Je is the 3rd person singular present of the verb biti = to be.

So:

• autobus je spor = the bus is slow

In Serbian, just like in English, you often need a form of to be when linking the subject to an adjective:

• On je visok. = He is tall.
• Vreme je lepo. = The weather is nice.
• Autobus je spor. = The bus is slow.

Because spor is an adjective, and here it describes the noun autobus.

• spor autobus = a slow bus
• autobus je spor = the bus is slow

By contrast, sporo is usually an adverb, meaning slowly:

• Autobus ide sporo. = The bus goes slowly.

So:

• spor describes what the bus is like
• sporo describes how something happens

Here pa means something like:

• so
• and so
• therefore
• which is why

In this sentence:

• Ovaj autobus je spor, pa uvek stiže kasno.
• This bus is slow, so it always arrives late.

It connects the two ideas in a natural, conversational way:

1. the bus is slow
2. because of that, it arrives late

It is very common in everyday Serbian.

Not exactly.

• pa links two clauses and often means so / and therefore
• zato often means therefore / that is why
• zato što means because

Compare:

• Autobus je spor, pa stiže kasno.
  = The bus is slow, so it arrives late.

• Autobus je spor, zato stiže kasno.
  = The bus is slow; therefore it arrives late.
  This is a bit more formal or emphatic.

• Autobus stiže kasno zato što je spor.
  = The bus arrives late because it is slow.

So in your sentence, pa is the most natural everyday connector.

Uvek means always.

So:

• uvek stiže kasno = always arrives late

Its position is flexible, but the placement here is very natural.

Common possibilities:

• uvek stiže kasno
• stiže uvek kasno
  less common / less natural in neutral speech
• stiže kasno uvek
  usually unnatural in ordinary neutral word order

A good basic pattern is:

• subject + uvek + verb + adverb
• Autobus uvek stiže kasno.

In your sentence, the subject was already mentioned, so the second clause simply starts with pa uvek...

Stiže means arrives / is arriving / gets there, depending on context.

In this sentence, it means arrives:

• uvek stiže kasno = it always arrives late

It is the 3rd person singular present form, matching autobus.

So:

• ja stižem = I arrive
• ti stižeš = you arrive
• on/ona/ono stiže = he/she/it arrives

Since autobus is singular, you use stiže.

This is a very useful question, because it involves aspect.

• stizati / stići are related verbs
• stiže comes from the imperfective verb stizati
• stigne comes from the perfective verb stići

In habitual or repeated situations, Serbian often prefers the imperfective:

• Ovaj autobus uvek stiže kasno.
  = This bus always arrives late.

That is why stiže sounds natural here: it describes something that happens regularly.

Stigne is more about reaching a result or a single completion:

• Kad stigne, javi mi. = When he arrives, let me know.
• Da li će autobus stići na vreme? = Will the bus arrive on time?

So:

• stiže = habitual/repeated arrival
• stigne / stići = one completed arrival, or focus on the result

Because kasno is an adverb, meaning late.

It describes how the bus arrives:

• stiže kasno = arrives late

The adjective kasan means late too, but it describes a noun, not the action:

• kasan autobus = a late bus
• kasan dolazak = a late arrival

So in this sentence:

• spor = adjective, describing autobus
• kasno = adverb, describing stiže

Yes, and that is exactly what happens here.

The sentence does not repeat the subject in the second clause:

• Ovaj autobus je spor, pa uvek stiže kasno.

Literally, English might expect:

• This bus is slow, so it always arrives late.

In Serbian, once the subject is clear, you usually do not need to repeat it. The verb form stiže already shows that the subject is he/she/it in the singular, and the context tells us it is the bus.

You could also say:

• Ovaj autobus je spor, pa autobus uvek stiže kasno.

But that sounds repetitive and less natural.

The word order can change somewhat, but the original sentence is very natural.

Standard version:

• Ovaj autobus je spor, pa uvek stiže kasno.

Possible variations:

• Ovaj autobus je spor, pa stiže kasno.
  = This bus is slow, so it arrives late.
• Ovaj autobus je spor, pa kasno stiže.
  Possible, but it puts more focus on late.
• Uvek stiže kasno.
  = It always arrives late.

Serbian word order is more flexible than English, but not random. Moving words often changes emphasis, not just style.

For a learner, the original order is a very good neutral model.

Then you would usually use a plural or a more general structure.

For example:

• Ovi autobusi su spori, pa uvek stižu kasno.
  = These buses are slow, so they always arrive late.

Or more generally:

• Autobusi su spori, pa često stižu kasno.
  = Buses are slow, so they often arrive late.

Your original sentence is about one specific bus:

• Ovaj autobus... = this bus

So the demonstrative ovaj makes it specific rather than general.