Ich wundere mich, dass der Bus heute pünktlich ist.

In German, sich wundern is usually a reflexive verb, so it normally comes with a reflexive pronoun:

• ich wundere mich
• du wunderst dich
• er wundert sich

So mich is not optional here. It is part of how the verb is normally used when you mean to be surprised or to wonder.

A good way to learn it is as a set phrase:

• sich wundern = to be surprised / to wonder

Without the reflexive pronoun, learners will usually sound incorrect or incomplete.

Mich is the accusative reflexive pronoun for ich.

So in:

• Ich wundere mich

the subject is ich, and the reflexive pronoun that matches it is mich.

Here is the reflexive pattern in the accusative:

• ich → mich
• du → dich
• er/sie/es → sich
• wir → uns
• ihr → euch
• sie/Sie → sich

So mich does not mean me in the usual object sense here. It is simply the reflexive form required by sich wundern.

Because dass introduces a subordinate clause, and in German subordinate clauses are separated from the main clause by a comma.

So the sentence has two parts:

• Ich wundere mich = main clause
• dass der Bus heute pünktlich ist = subordinate clause

German punctuation requires that comma. This is much stricter than in English.

Because dass sends the conjugated verb to the end of its clause.

Compare:

• Main clause: Der Bus ist heute pünktlich.
• After dass: ..., dass der Bus heute pünktlich ist.

This is one of the most important German word-order rules:

• In a main clause, the conjugated verb is usually in second position.
• In a subordinate clause introduced by words like dass, weil, obwohl, wenn, the conjugated verb usually goes to the end.

Because der Bus is the subject of the subordinate clause.

In the clause:

• dass der Bus heute pünktlich ist

the bus is the thing that is punctual, so it is the subject. Subjects are in the nominative case, and Bus is masculine, so the nominative form is der Bus.

If it were a direct object, you might see den Bus, but that is not what is happening here.

Yes, heute can move, but its position affects emphasis and naturalness.

In your sentence:

• ..., dass der Bus heute pünktlich ist

this is a very natural placement. Heute modifies the whole idea and sits in the middle of the clause.

Other possible versions include:

• ..., dass heute der Bus pünktlich ist
• ..., dass der Bus pünktlich ist heute — possible in speech, but less neutral

German allows some flexibility, but not complete freedom. The version with der Bus heute pünktlich ist sounds very normal and neutral.

Dass means that when introducing a clause.

So:

• Ich wundere mich, dass der Bus heute pünktlich ist.
• I’m surprised that the bus is on time today.

You use dass when one clause is the content of a thought, feeling, statement, or reaction:

• Ich weiß, dass ... = I know that ...
• Ich glaube, dass ... = I think that ...
• Es ist gut, dass ... = It is good that ...

English often drops that, but German usually keeps dass in this kind of sentence.

Not exactly.

Sich wundern usually means:

• to be surprised
• to be amazed
• to find something surprising

So in this sentence, Ich wundere mich, dass ... is closer to:

• I’m surprised that ...

It is usually not the best choice for the English meaning of I wonder if... / I wonder whether... when you mean I’m thinking about or I’m curious about something.

For example:

• I wonder whether he is coming.
  • more natural German: Ich frage mich, ob er kommt.

So:

• sich wundern = be surprised
• sich fragen = wonder / ask oneself

Yes, absolutely. That is a very natural alternative.

Compare:

• Ich wundere mich, dass der Bus heute pünktlich ist.
• Ich bin überrascht, dass der Bus heute pünktlich ist.

Both mean something like I’m surprised that the bus is on time today.

The version with überrascht may sound slightly more directly like surprised, while ich wundere mich can sometimes feel a bit more like I find it surprising. In many situations, though, they are very close.

Yes. You can put the subordinate clause first:

• Dass der Bus heute pünktlich ist, wundert mich.

This is grammatically correct, but it is a bit less neutral and often sounds more formal or stylistically marked.

Notice something important: in this version, the main clause changes from ich wundere mich to wundert mich. That is because now the subject of the main clause is the whole dass clause:

• Dass der Bus heute pünktlich ist = what is surprising
• wundert mich = surprises me

So this is not just a simple rearrangement of words; the structure changes slightly.

Because all nouns are capitalized in German.

So:

• der Bus
• heute
• pünktlich
• ich

Only Bus is a noun here, so it gets a capital letter.

This is one of the most visible spelling differences between German and English.

Here, pünktlich is an adjective used predicatively.

That means it comes after a form of sein and describes the subject:

• Der Bus ist pünktlich.

When adjectives are used this way in German, they do not take endings.

Compare:

• der pünktliche Bus → adjective before a noun, so it gets an ending
• der Bus ist pünktlich → adjective after sein, so no ending

So in your sentence, pünktlich stays in its basic form.