Nemini mendacium dicere licet.

Because licet normally takes the person affected in the dative case, not the nominative.

• nemo = no one as a subject
• nemini = to no one / for no one

So Latin literally says something like:

• To no one is it permitted to tell a lie.

That is why nemini is the correct form here.

Nemini is dative singular.

Its role is to show the person for whom something is allowed or not allowed. With licet, Latin often uses:

• dative of person
• infinitive of the action

So in Nemini mendacium dicere licet, nemini means for no one / to no one.

Because after licet, Latin commonly uses an infinitive to express what action is permitted.

So:

• licet = it is permitted / it is allowed
• dicere = to say / to tell

Together:

• dicere licet = it is permitted to say / to tell

This is a very common Latin construction:

• mihi ire licet = I am allowed to go
• tibi manere licet = you are allowed to stay

So dicere is infinitive because it names the action itself: to tell.

Because mendacium is the direct object of dicere.

• dicere means to say / tell
• what is being told? mendacium = a lie

So mendacium is in the accusative singular, because it is the thing being said or told.

Here licet means it is permitted, it is allowed, or more naturally in English, may or is allowed to.

It is an impersonal verb, which means it does not really have a normal personal subject like he or she. English often uses it in translation:

• licet = it is permitted

So the whole sentence is literally:

• To no one it is permitted to tell a lie.

More natural English:

• No one may tell a lie.
• No one is allowed to tell a lie.

Not in the usual way. Licet is impersonal, so Latin does not need a normal subject.

English often supplies it:

• it is permitted

But that it does not refer to anything concrete. It is just part of the English expression. Latin simply says licet without needing an expressed subject.

Latin word order is much more flexible than English word order, because the endings show each word’s role.

This order puts nemini first, which gives it emphasis:

• Nemini = to no one / no one

So the sentence strongly begins with the idea of no one.

A more literal order in English would be:

• To no one is it permitted to tell a lie.

Latin could rearrange the words and still keep the same basic meaning, as long as the grammar remains clear.

Yes. That is a perfectly natural translation.

Latin mendacium dicere literally means to tell/speak a lie, but English often expresses the same idea simply as:

• to lie

So these are all reasonable translations:

• No one may tell a lie.
• No one is allowed to tell a lie.
• It is permitted for no one to tell a lie. (more literal, less natural)
• No one is allowed to lie.

The dictionary forms are:

• nemo = no one
• mendacium, mendaciī = lie, falsehood
• dīcō, dīcere, dīxī, dictum = say, tell
• licet, licēre, licuit = it is permitted

A learner may notice that nemini does not look much like nemo. That is because nemo is irregular.

Yes. Nemini is singular.

Latin often uses singular forms like this where English also uses a singular idea:

• nemo = no one
• nemini = to no one

Even though the meaning applies to everybody, the grammar is singular because no one is grammatically singular.

Yes. Dīcere is a broad verb meaning to say, to speak, or to tell, depending on context.

With mendacium, English usually says:

• tell a lie or
• speak a lie

But tell a lie is much more natural English. So mendacium dicere is best understood as to tell a lie.

The main pattern is:

dative of person + infinitive + licet

Meaning:

someone is allowed to do something

Examples:

• mihi ire licet = I am allowed to go
• tibi loqui licet = you are allowed to speak
• nemini mendacium dicere licet = no one is allowed to tell a lie

This is a very useful pattern to remember, because it appears often in Latin.