Pater dicit nefas esse amicum fallere.

The sentence has two parts:

• Pater dicit = Father says
• nefas esse amicum fallere = the thing he says, in indirect statement form

So the structure is:

• pater = subject
• dicit = main verb
• nefas esse amicum fallere = that it is wrong to deceive a friend

A natural English translation uses a that-clause, but Latin usually expresses this idea differently.

After verbs like dicit (says), Latin very often uses an indirect statement. In this construction, the verb of the reported statement goes into the infinitive.

The direct statement would be:

• Nefas est amicum fallere.
• It is wrong to deceive a friend.

When that statement becomes indirect after dicit, est changes to esse:

• Pater dicit nefas esse amicum fallere.

So esse is simply the infinitive form corresponding to est.

Yes, broadly speaking, this is Latin indirect statement, which is often called the accusative-and-infinitive construction.

However, this sentence is slightly different from the most obvious textbook example.

A very typical example is:

• Pater dicit puerum venire.
• Father says that the boy is coming.

There, puerum is the accusative subject of venire.

But in Pater dicit nefas esse amicum fallere, the underlying statement is:

• Nefas est amicum fallere.
• It is wrong to deceive a friend.

This is an impersonal statement. There is no personal subject like the boy or Marcus. So there is no separate accusative subject here.

Important point:

• amicum is not the subject of fallere
• it is the object of fallere

Fallere is the present active infinitive of fallo, meaning to deceive.

Here it works like the English infinitive in to deceive a friend.

In the direct statement:

• Nefas est amicum fallere
• It is wrong to deceive a friend

the phrase amicum fallere functions as the idea being judged wrong.

So fallere stays as an infinitive because the sentence is talking about the action itself: deceiving a friend.

Because amicum is the direct object of fallere.

• fallere = to deceive
• whom do you deceive? amicum = a friend

So:

• amicum fallere = to deceive a friend

A learner may wonder whether amicum is accusative because of indirect statement, but here that is not the reason. It is accusative simply because fallere is a transitive verb and takes a direct object.

Nefas is a very important word to understand here.

It means something like:

• wrong
• forbidden
• against divine or moral law
• an impious thing

In this kind of sentence, nefas est means it is wrong or it is forbidden.

So:

• Nefas est amicum fallere = It is wrong to deceive a friend

A useful thing to know is that nefas is an indeclinable word. That means it does not change its form the way many Latin nouns and adjectives do.

In English we say:

• Father says that it is wrong to deceive a friend.

But in Latin, after verbs of saying, thinking, knowing, hearing, and so on, the language usually does not use a separate word meaning that. Instead, it uses the infinitive construction:

• dicit ... esse
• literally, says ... to be

So Latin expresses the reported idea through grammar rather than through a conjunction like that.

The direct statement is:

• Nefas est amicum fallere.
• It is wrong to deceive a friend.

Then, if you want to report that statement after pater dicit, you change est to esse:

• Pater dicit nefas esse amicum fallere.

This is a very good way to understand Latin indirect statement: first imagine the direct statement, then convert it.

Very literally, it is something like:

• Father says [it] to be wrong to deceive a friend.

That sounds awkward in English, but it helps show the Latin structure:

• Pater = Father
• dicit = says
• nefas esse = to be wrong
• amicum fallere = to deceive a friend

Natural English rearranges this into:

• Father says that it is wrong to deceive a friend.

Yes. Latin word order is much more flexible than English word order.

For example, the core relationships would stay the same even if the order changed, because the endings and forms show the grammar:

• Pater dicit amicum fallere nefas esse
• Nefas esse pater dicit amicum fallere

These all express basically the same idea, though some orders may sound more natural or more emphatic than others.

In your sentence, placing nefas esse before amicum fallere highlights the judgment first: that it is wrong...

Not always. Fallere can have a range of meanings, including:

• to deceive
• to trick
• to mislead
• sometimes to disappoint or escape the notice of

But in amicum fallere, the natural sense is to deceive or betray a friend.

So here deceive is the best basic translation.