Magistra dicit fas esse veritatem dicere, sed nefas esse mendacium amare.

Magistra is the subject of the sentence: the teacher, specifically a female teacher.

It is in the nominative singular because it is the one doing the action of dicit. The verb dicit is third person singular, so the grammar matches:

• magistra = the teacher
• dicit = says

So the sentence begins with The teacher says...

This is a very common Latin pattern. After verbs like say, think, know, and hear, Latin often uses an infinitive construction where English uses that + a normal verb.

So instead of something like:

• The teacher says that it is right...

Latin says:

• Magistra dicit fas esse...

A useful way to see it is to compare the direct and reported versions:

• direct: Fas est veritatem dicere.
• reported: Magistra dicit fas esse veritatem dicere.

So est becomes esse when the statement is reported after dicit.

Fas and nefas are special Latin words meaning something like:

• fas = what is right, permitted, lawful in a moral or divine sense
• nefas = what is wrong, forbidden, impious

They are not ordinary adjectives like bonus or malus. They are often used in impersonal expressions:

• fas est = it is right / it is permitted
• nefas est = it is wrong / it is forbidden

There is often a slightly stronger sense than simple personal preference. Fas can suggest what is right according to divine or sacred order, not just what someone happens to like.

Because the sentence contains two different layers.

In fas esse veritatem dicere:

• esse belongs with fas: to be right
• dicere names the action being judged: to speak / to tell

So the structure is basically:

• [it] to be right [to tell the truth]

Likewise in nefas esse mendacium amare:

• esse = to be
• amare = to love

So Latin is saying:

• the teacher says it is right to tell the truth
• but wrong to love a lie / falsehood

Because veritatem is the object of dicere.

• veritas = truth, as a subject form
• veritatem = truth, as an object form

Latin uses dicere with an object here, so veritatem dicere means:

• to speak the truth
• to tell the truth

So veritatem is not the subject of the clause. The whole infinitive phrase veritatem dicere acts as the thing being judged by fas.

Mendacium is the object of amare, so it is accusative for the same reason that veritatem is accusative after dicere.

• amare mendacium = to love a lie / to love falsehood

As for the singular, Latin often uses the singular to express a general idea. So mendacium here does not have to mean just one specific lie. It can mean something broader, like:

• lying
• falsehood
• what is false

English might sometimes prefer lies or falsehood, but Latin singular is perfectly natural here.

Because the sentence is making a general statement, not talking about one specific person doing the action.

So:

• veritatem dicere = telling the truth
• mendacium amare = loving falsehood / loving a lie

This works much like English gerund phrases such as telling the truth is right.

Latin infinitives can do this very naturally. The idea is not she tells the truth or he loves lying, but rather the action itself in a general sense.

That is a very good question. Often, yes, Latin indirect statement uses an accusative subject + infinitive, as in:

• dicit puerum venire = he says that the boy is coming

There, puerum is the accusative subject of venire.

But in this sentence, the reported content is built around the impersonal expressions fas est and nefas est. In the direct form, you already have:

• Fas est veritatem dicere
• Nefas est mendacium amare

Here the infinitive phrases veritatem dicere and mendacium amare function as the subject-like ideas of fas est and nefas est. There is no separate personal subject such as he, she, or the boy. So when the sentence is reported after dicit, there is no accusative subject to appear.

So this is still a normal Latin reported construction, but not the kind that has a separate accusative person as subject.

Sed simply means but, and it joins the two parallel ideas:

• fas esse veritatem dicere
• nefas esse mendacium amare

It stands between them because it is contrasting the two judgments:

• one thing is right
• the other is wrong

This gives the sentence a neat balance, which Latin often likes.

Latin word order is much more flexible than English word order. This sentence is a perfectly natural arrangement, but other orders are also possible.

For example, Latin could also say:

• Magistra dicit veritatem dicere fas esse
• Magistra dicit mendacium amare nefas esse

Those versions would mean essentially the same thing.

The order in your sentence puts fas and nefas early in each clause, which gives emphasis to the moral judgment itself:

• right
• wrong

So the chosen order is not random; it helps highlight the contrast.

Because dicere veritatem is a straightforward and idiomatic Latin way to express to tell the truth or to speak the truth.

Literally it is:

• dicere = to say, to speak
• veritatem = the truth

Latin often uses a simple verb + object where English may use a slightly more specialized phrase. So even if English learners expect something like tell truthfully, Latin is very comfortable with say the truth as its normal expression.

In other words, this is not strange Latin at all; it is a natural way to say the idea.