Controllo l’indirizzo sul foglietto prima di uscire.

Controllo is the first-person singular present tense form of controllare.

• controllare = to check
• controllo = I check / I am checking

So the sentence uses a fully conjugated verb, not the infinitive.

Italian often leaves out subject pronouns when they are not needed.

Because controllo already means I check, adding io would usually only be for emphasis or contrast.

• Controllo l’indirizzo... = I check the address...
• Io controllo l’indirizzo... = I check the address... / As for me, I check the address...

Because indirizzo begins with a vowel, il becomes l’ before it.

• il
  • consonant: il libro
• l’
  • vowel: l’indirizzo

This is called elision.

Here indirizzo means address, usually a street address or written destination.

It does not mean speech here, even though in some contexts indirizzo can have more formal meanings. In an everyday sentence like this, the most natural meaning is simply address.

Sul is a contraction of su + il.

• su = on
• il = the
• sul = on the

So sul foglietto literally means on the little sheet / on the slip of paper.

Foglietto is a diminutive of foglio.

• foglio = sheet of paper
• foglietto = little sheet of paper, small note, slip of paper

The ending -etto often gives the idea of something small or sometimes a bit informal or affectionate. Here it suggests a small piece of paper or note.

After prima di, Italian normally uses the infinitive when the subject is the same as the subject of the main verb.

So:

• Controllo l’indirizzo prima di uscire. = I check the address before leaving.

Both actions are done by the same person: I check, and I leave.

If the subject changes, Italian usually uses a clause instead:

• Controllo l’indirizzo prima che lui esca. = I check the address before he leaves.

It can mean both, depending on context.

• uscire = to go out, to exit, to leave

In this sentence, prima di uscire most naturally means before leaving / before going out. English could translate it in more than one way depending on the situation.

It can be understood either way, because the Italian present tense is flexible.

Depending on context, controllo can mean:

• I check
• I’m checking

And it can describe:

• a habitual action: I check the address before leaving
• a current action: I’m checking the address before leaving

Without extra context, both are possible.

Because l’indirizzo is the direct object of controllo.

You check something, so in Italian the thing being checked comes directly after the verb:

• Controllo l’indirizzo = I check the address

No extra word like a or di is needed.

Usually sul foglietto literally means on the little piece of paper or on the note, because the address is written on the paper.

English sometimes says in the note depending on the context, but Italian su focuses on the physical surface of the paper. So on the slip of paper is the closest match.