Nuntius renuntiat pontem prope portum cras refectum iri, itaque plaustra hodie exspectare debere.

The main clause is:

• Nuntius renuntiat = The messenger reports

After that, Latin gives the content of the report in indirect statement form. In fact, there are two reported ideas:

1. pontem prope portum cras refectum iri
   = that the bridge near the harbor will be repaired tomorrow

2. itaque plaustra hodie exspectare debere
   = and so that the wagons should wait today

So the sentence is basically:

• The messenger reports [that X], and therefore [that Y].

Because Latin usually does not introduce indirect statements with a word like English that.

After verbs of saying, thinking, knowing, hearing, reporting, and so on, Latin commonly uses the accusative-and-infinitive construction:

• renuntiat pontem refectum iri
• literally: he reports the bridge to be going to be repaired
• natural English: he reports that the bridge will be repaired

So instead of that + finite verb, Latin normally uses:

• an accusative subject
• plus an infinitive

Because pontem is the subject of an indirect statement.

In English we say:

• that the bridge will be repaired

In Latin, in indirect statement, the subject becomes accusative:

• pontem ... refectum iri

So although the bridge is logically the subject of will be repaired, Latin puts it in the accusative because it is inside reported speech.

refectum iri is the future passive infinitive of reficere (to repair, restore).

So:

• refectum iri = to be going to be repaired
• smoother English: will be repaired

In this sentence:

• pontem ... cras refectum iri
• that the bridge ... will be repaired tomorrow

A useful point for learners: Latin does not have a simple one-word future passive infinitive here, so it uses this special form.

Usually this form is analyzed as the supine in -um plus iri:

• refectum iri

So strictly speaking, refectum here is not just an ordinary participle agreeing with pontem, even though it looks exactly like one.

That said, for a learner reading the sentence, the important thing is:

• pontem ... refectum iri functions together as
  the bridge will be repaired

So even if the form looks like agreement, the standard grammatical explanation is that refectum iri is a fixed future passive infinitive formation.

Because prope takes the accusative.

So:

• prope portum = near the harbor

This is normal Latin usage:

• prope urbem = near the city
• prope flumen = near the river

Because it is still part of the reported statement after renuntiat.

Just as Latin says:

• pontem ... refectum iri

it also says:

• plaustra ... exspectare debere

So the messenger reports not only the future repair, but also the consequence:

• that the wagons ought to wait today

If Latin used debent, that would make it a new finite clause outside the indirect statement. But here the idea is still being reported by the messenger, so Latin keeps the infinitive:

• debere = to ought / to have to, i.e. ought / must in context

Yes: the subject of an indirect statement should be accusative.

The reason plaustra looks surprising is that plaustrum is a neuter noun. In Latin, neuter plural nominative and accusative have the same form:

• nominative plural: plaustra
• accusative plural: plaustra

So here plaustra is functioning as the accusative subject of exspectare debere, even though its form looks the same as the nominative.

Because the second indirect statement needs a subject, and plaustra fits that role:

• plaustra ... exspectare debere
• that the wagons ought to wait

Also, debere naturally goes with a subject plus infinitive:

• the wagons ought to wait

So in this sentence, plaustra is best taken as the accusative subject of the infinitive phrase.

itaque means and so, therefore, accordingly.

It connects the second reported idea to the first:

• the bridge will be repaired tomorrow,
• therefore the wagons should wait today.

So the logic is:

1. repair is happening tomorrow,
2. therefore waiting is necessary today.

Latin often does not repeat the reporting verb, so after renuntiat you understand it with both parts.

They modify different parts of the reported statement:

• cras goes with refectum iri
  = will be repaired tomorrow

• hodie goes with exspectare debere
  = should wait today

So the message distinguishes two times:

• the repair happens tomorrow
• the waiting happens today

Yes, exspectare very often means to wait for, expect.

But in context it can also be understood more simply as to wait. Here the sense is:

• plaustra hodie exspectare debere
• the wagons should wait today

So the idea is not that the wagons are waiting for a direct object named in the sentence, but that they are to remain waiting / hold off for now.

Yes, Latin could repeat the reporting verb, but it often does not need to.

So instead of writing something like:

• Nuntius renuntiat ... et renuntiat ...

Latin simply gives the first reporting verb once, and the second indirect statement is understood as still depending on it.

That makes the sentence more compact:

• Nuntius renuntiat [X], itaque [Y].

In other words:

• The messenger reports [X], and therefore [Y].