Sed magistra monet neminem celeriter iudicare oportere, antequam totam fabulam audiverit.

The sentence has three parts:

1. Sed magistra monet
   This is the main clause: But the teacher warns / advises / reminds.

2. neminem celeriter iudicare oportere
   This is the content of what she warns, expressed as an indirect statement.

3. antequam totam fabulam audiverit
   This is a temporal clause with antequam, explaining before hearing the whole story.

So the skeleton is:

the teacher warns + that no one ought to judge quickly + before hearing the whole story.

Because Latin is using an accusative-and-infinitive construction for indirect statement.

In English, we often say:

• She warns that no one ought to judge quickly

In Latin, instead of a that-clause, you often get:

• monet neminem ... oportere

In this construction, the subject of the indirect statement goes into the accusative, so nemo becomes neminem.

So:

• nemo = no one, nobody
• neminem = no one, nobody, in the accusative

Here neminem is the understood subject of iudicare within the indirect statement: that no one should judge quickly.

Because Latin very often does not use a separate word equivalent to English that in indirect statement.

Instead of:

• She warns that no one ought...

Latin prefers:

• She warns no one to-ought...
• more naturally understood as
  She warns that no one ought...

The pattern is:

• verb introducing a statement
• accusative subject
• infinitive verb

So monet neminem ... oportere is Latin’s normal way of expressing the idea that English gives with that.

Because the sentence is layering one idea inside another.

• oportere = to be proper / to be necessary / ought
• iudicare = to judge

In the indirect statement, oportere is the main infinitive:

• neminem ... oportere = that no one ought

But oportere itself needs another verb to complete the idea:

• iudicare oportere = ought to judge

So the structure is basically:

• monet = she warns
• neminem ... oportere = that no one ought
• iudicare = to judge

A very literal unpacking would be something like:

• the teacher warns that it is proper/necessary for no one to judge quickly

More natural English:

• the teacher warns that no one ought to judge quickly

Oportere is the infinitive of oportet, an impersonal verb meaning something like:

• it is proper
• it is right
• it is necessary
• one ought

In this sentence, the best natural English is usually:

• ought
• should

So neminem celeriter iudicare oportere means:

• that no one ought to judge quickly
• or that one should not judge quickly

It is a little more formal and impersonal than some other ways Latin can express obligation.

It modifies iudicare.

So:

• celeriter iudicare = to judge quickly

Here celeriter is an adverb, formed from celer meaning swift/quick.

In sense, it often implies not just speed, but hasty judgment. So the idea is not merely physical quickness, but judging too quickly or rashly.

This is a very common point of confusion.

The form audiverit can look tricky because it is formally identical to:

• future perfect indicative
• and perfect subjunctive

In this sentence, the sense is clearly prospective: the hearing must happen first, before the judging.

So the idea is:

• before someone has heard the whole story
• or before someone will have heard the whole story

In smoother English, we usually just say:

• before hearing the whole story
• before one has heard the whole story

Latin often uses this kind of form after antequam when it is looking ahead to an action that must be completed before another action happens.

It goes with the idea of judging, not with the idea of warning.

So the meaning is:

• The teacher warns that no one ought to judge quickly before hearing the whole story

not:

• The teacher warns before she has heard the whole story

In other words, the before-clause limits when judging should happen, not when the teacher gives the warning.

It is not stated explicitly, but it is understood to be the same general person referred to by neminem.

So the sense is:

• before anyone has heard the whole story
• or before one has heard the whole story

Latin often leaves a repeated subject unstated when it is clear from context.

Because fabulam is the direct object of audiverit:

• audire fabulam = to hear a story

And totam agrees with fabulam:

• feminine
• singular
• accusative

So:

• fabulam = story
• totam fabulam = the whole story

This is also very natural idiomatic Latin, just as English says hear the whole story.

The word order is meaningful, but Latin is much freer than English.

English needs a fairly fixed order:

• that no one ought to judge quickly

Latin can move parts around more easily because the endings show the grammar.

Here:

• neminem is placed early, giving prominence to no one
• celeriter sits right next to iudicare, which makes sense because it modifies it
• oportere comes at the end of the indirect statement, where Latin often likes to place an infinitive or important verbal idea

So the order is natural Latin, even though a word-for-word English rearrangement would sound awkward.