Post comitia magistra discipulis refert quid in curia de novo magistratu dictum sit.

Post is a preposition that takes the accusative. So comitia here is accusative plural: after the assembly / after the elections.

A useful thing to know is that comitia is a neuter plural word, and its nominative and accusative forms look the same. So the form itself does not change, but after post you know it must be accusative.

Also, comitia is often used in the plural in Latin even when English may use a more singular-sounding translation.

Because magistra is the only word in the sentence that naturally works as a nominative singular subject with refert, which is third person singular: she reports.

So the core of the sentence is:

magistra ... refert
= the teacher ... reports

Latin relies on endings more than English does, so you look for a nominative noun that matches the verb.

Because discipulis is the indirect object: the person to whom something is reported.

With referre in this sense, Latin commonly uses:

• aliquid referre = to report something
• alicui referre = to report to someone

So:

magistra discipulis refert
= the teacher reports to the students

Yes, it is related, but here it means reports, relates, or tells.

The verb is refero, referre, rettuli, relatum, literally something like bring back, but in many contexts it develops the sense bring back a report, hence report or relate.

So in this sentence refert does not mean modern English refer to. It means she reports/tells.

Because quid introduces an indirect question and means what.

So:

quid ... dictum sit
= what ... was said

By contrast, quod is usually:

• a relative pronoun: which / that
• or a conjunction: because, sometimes that

Here the sentence is asking about content in an indirect-question form: what was said, not that something was said. So quid is the right word.

Because both prepositions here govern the ablative:

• in
  • ablative = in, on when it shows location
• de
  • ablative = about, concerning

So:

• in curia = in the senate-house / council chamber
• de novo magistratu = about the new magistrate

If in meant motion into a place, it would take the accusative instead.

Because magistratus is a fourth-declension noun.

Its ablative singular ends in -u, so:

• nominative singular: magistratus
• ablative singular: magistratu

That is why after de you get de novo magistratu.

The adjective novo matches it in case, number, and gender, so both are ablative singular.

Dictum sit is the perfect passive subjunctive of dico, dicere.

It is made from:

• dictum = the perfect passive participle
• sit = the subjunctive of sum

So literally it means it has been said.

In this sentence it is used impersonally, so Latin is not saying who said it. The focus is only on the content: what was said.

Because quid in curia de novo magistratu dictum sit is an indirect question, and Latin normally puts the verb of an indirect question in the subjunctive.

Compare:

• direct question: quid dictum est? = what was said?
• indirect question: refert quid dictum sit = she reports what was said

So the subjunctive here does not make the meaning doubtful. It is simply the normal grammar of an indirect question.

Because the saying happened before the reporting.

The main verb refert is present: she reports.
Inside the indirect question, Latin uses perfect subjunctive to show an action completed earlier: what has been said / what was said.

So the tense relationship is:

• refert = present reporting
• dictum sit = earlier completed speaking

This is a standard use of the perfect subjunctive in an indirect question after a primary tense.

Latin word order is much more flexible because the endings show the grammatical relationships.

English depends heavily on position:

• the teacher tells the students

Latin can move words around more freely because:

• magistra shows the subject
• discipulis shows the indirect object
• quid ... dictum sit is clearly the indirect question

So the sentence can be understood by its forms, not just by its order. The order here lets Latin place the reported-content clause naturally at the end:

Post comitia magistra discipulis refert quid in curia de novo magistratu dictum sit.

That is a very normal Latin arrangement.