Danas je veliki saobraćaj u centru, pa autobus ide sporije nego obično.

Because je is a clitic in Serbian. Clitics are short unstressed words that usually go in the second position of the sentence or clause.

So:

• Danas je veliki saobraćaj... = natural
• not usually Danas veliki je saobraćaj...

Serbian word order is more flexible than English, but clitic placement is one of the big things that still has rules.

You can also move other parts around, for example:

• U centru je danas veliki saobraćaj.
• Veliki je saobraćaj u centru danas.

But je still tries to stay in that early clitic position.

Serbian often uses to be where English uses there is / there are.

So:

• Danas je veliki saobraćaj u centru
  literally looks like Today is big traffic in the center
• but naturally it means There is heavy traffic downtown/in the city center today

This is normal Serbian structure. English needs there is, but Serbian often does not.

Here veliki saobraćaj means heavy traffic.

Word by word:

• veliki = big, large
• saobraćaj = traffic

Serbian often uses big/large in places where English prefers heavy. So you should understand it idiomatically, not too literally.

Also:

• saobraćaj is a masculine singular noun
• that is why the adjective is veliki and not some other form

No, in this meaning it works like a mass noun, much like English traffic.

You do not usually count individual traffics in English, and Serbian works similarly with saobraćaj. So:

• veliki saobraćaj = heavy traffic
• not something like many traffics

That is why the verb is singular too:

• je veliki saobraćaj

Because u can take different cases depending on the meaning:

• u + locative = location, in / at
• u + accusative = direction, into

Here the meaning is location:

• u centru = in the center / downtown

The base noun is centar, and after u for location it becomes centru.

Compare:

• u centru = in the center
• u centar = into the center

Here pa means so, therefore, or and as a result.

In this sentence:

• Danas je veliki saobraćaj u centru, pa autobus ide sporije nego obično.
• heavy traffic exists, so the bus goes more slowly than usual

Pa is very common in everyday Serbian and can have a few shades of meaning depending on context. Here it expresses a consequence.

Because pa is joining two clauses here:

• Danas je veliki saobraćaj u centru
• autobus ide sporije nego obično

In Serbian, a comma is normally used before pa when it links clauses like this.

So the punctuation is standard and natural.

Yes. In Serbian, ići is very commonly used for vehicles too.

So:

• autobus ide = the bus goes / is going / runs

This is perfectly normal.

You may also see related verbs:

• voziti = to drive, to transport
• voziti se = to ride, to travel by vehicle

But in this sentence, ide is simple and natural for a bus moving along its route.

Because sporije is an adverb comparative, while sporiji is an adjective comparative.

Here the word describes how the bus goes:

• autobus ide sporije = the bus goes more slowly

Since it modifies the verb ide, you need the adverb.

Compare:

• spor autobus = a slow bus
• sporiji autobus = a slower bus
• autobus ide sporo = the bus goes slowly
• autobus ide sporije = the bus goes more slowly

It comes from the adverb sporo = slowly.

The comparative is:

• sporo → sporije = more slowly

This is a very common pattern in Serbian. A few examples:

• brzo → brže = more quickly
• tiho → tiše = more quietly
• sporo → sporije = more slowly

So in the sentence, sporije simply marks comparison.

Nego obično means than usual.

Here:

• nego = than
• obično = usually, ordinarily

So:

• autobus ide sporije nego obično
  = the bus goes more slowly than usual

In this sentence, obično functions adverbially. You can think of it as an abbreviated comparison:

• sporije nego što obično ide
  = more slowly than it usually goes

But Serbian very often shortens this to nego obično.

Usually, no — not in this sentence.

The natural expression is:

• sporije nego obično

With comparatives, Serbian often uses:

• od before a noun or pronoun
  
  • brži od mene = faster than me
• nego when the comparison is more clause-like or adverbial
  
  • brži nego što sam mislio = faster than I thought
  • sporije nego obično = more slowly than usual

So here nego is the idiomatic choice.

Because Serbian has no articles.

English says:

• the center
• a bus
• the bus

Serbian simply says:

• centar / centru
• autobus

Whether something is definite or indefinite is understood from context.

So in this sentence, English needs articles, but Serbian does not.

A simple guide:

• saobraćaj ≈ sa-o-bra-chaiy, with a soft ć
• obično ≈ obeechno, with a hard č

The important difference is:

• ć = softer sound
• č = harder sound, like ch in church

So:

• saobraćaj has ć
• obično has č

Also, saobraćaj has all three vowels pronounced separately at the start:

• sa-o-bra-ćaj

not one blended sound.