Rea veniam petit et dicit se iam mendacii paenitere.

It can be understood either way depending on context, but in a textbook sentence like this, Rea is very likely a proper name and the subject of the sentence.

Grammatically, it is in the nominative singular, which fits the verbs petit and dicit:

• Rea veniam petit = Rea asks for pardon
• Rea ... dicit = Rea says ...

If your text has already given a translation with a personal name, then that is the right way to take it here.

This is a very common Latin expression.

• veniam = pardon, forgiveness, favor
• petit = seeks, asks for, requests

So veniam petere literally means to seek pardon, and more naturally in English, to ask for forgiveness or to beg pardon.

Notice that veniam is in the accusative, because it is the direct object of petit.

Because Latin usually does not use a separate word like English that after verbs of saying, thinking, knowing, and perceiving.

Instead, Latin normally uses the accusative-and-infinitive construction (often called indirect statement):

• dicit = she says
• se ... paenitere = that she regrets ...

So:

• dicit se iam mendacii paenitere
  = she says that she now regrets the lie

This is one of the most important Latin constructions to learn.

Because se is the reflexive pronoun, and it refers back to the subject of the main verb, here Rea.

So in:

• Rea dicit se ... paenitere

the meaning is:

• Rea says that she herself regrets ...

If Latin used eam, that would normally mean her as a different female person, not Rea herself.

So:

• se = herself / herself as the subject of the indirect statement
• eam = her, someone else

Because it is part of the indirect statement after dicit.

After a verb like dicit, Latin often uses:

• an accusative subject +
• an infinitive

Here:

• se is the accusative subject
• paenitere is the infinitive

So literally the structure is something like:

• Rea says herself now to regret the lie

That is not natural English, so we translate it as:

• Rea says that she now regrets the lie

Because paenitere has a special construction.

The verb paenitet / paenitere means to cause regret or to make someone sorry, and it commonly takes:

• the person regretting in the accusative
• the thing regretted in the genitive

So in:

• se iam mendacii paenitere

we have:

• se = the person who feels regret
• mendacii = the thing regretted, of the lie

This is not the same pattern English uses, so it often feels strange at first.

It helps to think of it this way:

• paenitere is the infinitive of an impersonal-style verb
• the person affected goes into the accusative
• the thing regretted goes into the genitive

So:

• se mendacii paenitere

literally means something like:

• that it repents her of the lie
• or that she feels regret for the lie

In smoother English, we simply say:

• that she regrets the lie

It can mean either now or already, depending on context.

In this sentence, iam most naturally means now:

• she says that she now regrets the lie

But the idea of already may also be present in some contexts:

• she says that she already regrets the lie

Both senses are common for iam. Your translation should follow the context given in your text.

Latin word order is more flexible than English word order, because the endings show how the words function.

So the sentence could be rearranged in various ways without changing the basic meaning. The given order is perfectly normal:

• dicit introduces the indirect statement
• se gives the subject of that indirect statement
• iam adds the time idea
• mendacii goes with paenitere

The order may also help emphasize certain words, but grammatically the meaning comes mainly from the forms:

• se = accusative
• mendacii = genitive
• paenitere = infinitive

Because the speaker is describing actions happening in the present of the story:

• petit = asks
• dicit = says
• paenitere in the indirect statement is understood as simultaneous with dicit

So the idea is:

• Rea asks for pardon and says that she now regrets the lie

If the main verb were past, Latin could still use the infinitive in the indirect statement, but the time relationship would then be understood from the context and the tense of the infinitive construction.

A very literal translation would be:

• Rea seeks pardon and says herself now to regret the lie.

That is good for seeing the grammar, but not good English.

A natural English translation is:

• Rea asks for forgiveness and says that she now regrets the lie.

Or, depending on context:

• Rea begs pardon and says that she is now sorry for the lie.

That is often the best approach with Latin: first understand the structure literally, then turn it into normal English.