Ich weiß nicht, wieso der Bus heute zu spät kommt.

Because wissen is the infinitive, and here it has to be conjugated for ich.

• infinitive: wissen = to know
• ich weiß = I know

This verb is a little irregular in the present tense:

• ich weiß
• du weißt
• er/sie/es weiß
• wir wissen
• ihr wisst
• sie/Sie wissen

Also, the spelling weiß uses ß, not ss, in standard German spelling.

Wieso means why. In this sentence, it works very much like warum.

So these are both natural:

• Ich weiß nicht, wieso der Bus heute zu spät kommt.
• Ich weiß nicht, warum der Bus heute zu spät kommt.

In everyday German, wieso, warum, and sometimes weshalb can all mean why, though warum is probably the most neutral and common choice for learners to know first.

Because wieso der Bus heute zu spät kommt is a subordinate clause, specifically an embedded question.

In German, the conjugated verb usually goes to the end of a subordinate clause.

Compare:

• direct question: Wieso kommt der Bus heute zu spät?
• embedded question: Ich weiß nicht, wieso der Bus heute zu spät kommt.

So after wieso, the clause does not use normal question word order. It uses subordinate-clause word order, with the verb at the end.

Because der Bus is the subject of the clause der Bus heute zu spät kommt.

Ask yourself: Who is coming late?
Answer: the bus

That makes the bus the subject, so it stays in the nominative case:

• der Bus = nominative masculine singular

If it were a direct object, then you might see den Bus, but not here.

Here, nicht negates the verb weiß:

• Ich weiß nicht ... = I do not know ...

So the structure is:

• Ich weiß nicht
  • embedded question

You are not negating the bus or today or late. You are negating knowing.

That is why nicht comes before the subordinate clause rather than inside it.

zu spät means too late or, in this context, late in the sense of later than expected / later than it should be.

• spät = late
• zu spät = too late / late

With transportation, German often uses zu spät kommen for to be late:

• Der Bus kommt zu spät. = The bus is late.

Even though zu often means too, this phrase is the normal German way to express lateness in many contexts.

German often expresses arrival timing with kommen:

• Der Bus kommt zu spät. = literally The bus comes too late
• natural English meaning: The bus is late

So German focuses on the bus arriving/coming later than expected, where English often uses to be late.

This is a very common pattern:

• Der Zug kommt pünktlich. = The train is on time.
• Das Essen kam spät. = The food arrived late.

It is an embedded or indirect question, not a standalone direct question.

Direct question:

• Wieso kommt der Bus heute zu spät?

Embedded question:

• Ich weiß nicht, wieso der Bus heute zu spät kommt.

So yes, it contains the meaning of a question, but grammatically it is part of a larger sentence.

Heute means today, and it is an adverb of time.

In this sentence:

• wieso der Bus heute zu spät kommt

it comes before zu spät, which is very natural.

You may also hear slightly different word orders depending on emphasis, for example:

• ..., wieso heute der Bus zu spät kommt.
• ..., wieso der Bus zu spät kommt heute. (less neutral)

The version in your sentence is the most natural neutral order for everyday German.

weiß is pronounced roughly like English vice, but with a German w, which sounds like English v.

So approximately:

• weiß → vice

About ß:

• ß is called Eszett or scharfes S
• it represents an s sound, not a z sound
• so weiß sounds like vaiss / vice, not like wayz

Also note:

• w in German usually sounds like English v
• ei in German usually sounds like English eye

Yes, that is understandable and natural.

Both are possible:

• Ich weiß nicht, wieso der Bus heute zu spät kommt.
• Ich weiß nicht, warum der Bus heute zu spät ist.

There is a small nuance:

• zu spät kommt focuses more on the bus arriving/coming late
• zu spät ist focuses more on the bus being late

In many everyday situations, the difference is small, and both would be understood easily.

It is neutral everyday German.

Nothing in it is especially formal or especially casual.

• Ich weiß nicht = standard
• wieso = common in everyday speech
• der Bus heute zu spät kommt = standard grammar

If you wanted a slightly more formal feel, warum might sound a bit more neutral than wieso, but both are perfectly normal.

As a direct question, the word order changes:

• Wieso kommt der Bus heute zu spät?

Notice the difference:

• embedded question: ..., wieso der Bus heute zu spät kommt
• direct question: Wieso kommt der Bus heute zu spät?

In a direct question, the conjugated verb kommt moves to the second position. In the embedded question, it goes to the end.