Rileggi il messaggio prima di inviarlo.

It is the informal singular imperative of rileggere (to reread / to read again).

So the sentence is giving a command to tu:

• (tu) rileggi = reread!

A useful thing to know is that this form looks exactly like the present tense form:

• tu rileggi = you reread / you are rereading
• Rileggi! = Reread!

The context tells you which meaning it has. Here, because it is an instruction, it is the imperative.

Because rileggere means to reread or to read again, while leggere just means to read.

The prefix ri- often means again in Italian:

• leggere = to read
• rileggere = to reread

Italian often uses a prefixed verb where English might use a separate word like again.

So these are similar in meaning:

• Rileggi il messaggio.
• Leggi di nuovo il messaggio.

Both are natural, but rileggi is compact and very common.

Italian uses articles more often than English does. In this sentence, il messaggio is the direct object, and using the definite article is completely normal.

So:

• il messaggio = the message

Even when English might be flexible, Italian usually wants the article here.

If the speaker wanted a different nuance, they could change it:

• un messaggio = a message
• questo messaggio = this message
• quel messaggio = that message

But in your sentence, il messaggio is the standard, natural choice.

Prima di means before when it is followed by a verb in the infinitive.

So the pattern is:

• prima di + infinitive

Examples:

• prima di uscire = before going out
• prima di parlare = before speaking
• prima di inviarlo = before sending it

This structure is especially used when the subject is the same for both actions. In your sentence, the person who rereads is also the person who sends.

If the subject changes, Italian usually prefers prima che + subjunctive:

• Rileggi il messaggio prima che Luca lo invii. = Reread the message before Luca sends it.

Because lo is an unstressed direct object pronoun, and with an infinitive it attaches to the end of the verb.

So:

• inviare + lo → inviarlo

This is the normal rule with infinitives:

• farlo = to do it
• vederlo = to see it
• scriverlo = to write it
• inviarlo = to send it

So prima di inviarlo is correct, while prima di inviare lo is not.

It refers back to il messaggio.

Lo is a direct object pronoun meaning him/it, and here it means it because it stands for the message.

Since messaggio is masculine singular, the pronoun is lo:

• il messaggio → lo

Compare:

• il libro → lo
• la lettera → la
• i messaggi → li
• le lettere → le

So inviarlo literally means to send it.

Italian, like English, often uses a pronoun to avoid repeating the same noun.

So instead of saying:

• Rileggi il messaggio prima di inviare il messaggio

Italian naturally says:

• Rileggi il messaggio prima di inviarlo

This sounds smoother and more natural. Repeating il messaggio would usually sound clumsy unless the speaker wanted extra emphasis or clarity.

Yes. That is completely correct.

Both of these are natural:

• Rileggi il messaggio prima di inviarlo.
• Prima di inviarlo, rileggi il messaggio.

The meaning is basically the same. The difference is mainly one of emphasis:

• starting with Rileggi... puts the command first
• starting with Prima di inviarlo... emphasizes the timing/condition first

So the word order is flexible here.

Yes. As written, it is addressed to one person informally (tu).

Other versions would be:

• Rileggi il messaggio prima di inviarlo. = informal singular
• Rilegga il messaggio prima di inviarlo. = formal singular (Lei)
• Rileggete il messaggio prima di inviarlo. = plural (voi)

So the rest of the sentence can stay the same, while the imperative verb changes depending on who you are addressing.

Yes, in isolation it could, because the form is the same.

For example:

• Tu rileggi il messaggio prima di inviarlo. = You reread the message before sending it.

But in your sentence, there is no subject pronoun, and the sentence functions as an instruction. That makes the imperative reading the natural one:

• Rileggi il messaggio prima di inviarlo. = Reread the message before sending it.

In Italian, commands often omit the subject pronoun, just like English often does.

You may sometimes see elision in Italian, so d’inviarlo is possible in some styles, but for learners the safest and most standard form is:

• prima di inviarlo

That is the form you should actively use. It is clear, correct, and completely natural.