"Usare" and "Utilizzare" have the same meaning, there is no difference.
- "Usi" (verb) is used in two cases:
1) "Tu usi il coltello" = "You use the knife" : present indicative.
2) "Usi il coltello!" = "Use the knife!" : present imperative. In italian is a formal form, when you say to a person "Lei usi il coltello". You are studying Spanish too: well, it's similar to usted/ustedes when used as a formal form.
- "Usa" is used in two cases too:
1) "Lui usa il coltello" = "He uses the knife" : present indicative.
2) "Usa il coltello" = "Use the knife!" : present imperative. If your Italian girlfriend is using a spoon to cut the bread ( Wtf? ahah ) you say to her: Usa il coltello!
Remember the Second Person "Lei" used as a formal form for "You".
Okay, but those examples sound pretty similar to me and I couldn't tell whether to use "usi" or "usa". At first I thought "usa" is the third person singular indicative and "usi" the second person singular indicative". But if I want someone to use something, do I say "usa" or "usi"? I can't really tell by the examples... :(
It can be different for other verbs:
"Ehi andrea, senti" = "Listen to me"
"Signore, senta" : this is the formal "You", in italian "Lei"
"Andrea, esci" = "Go out"
"Signore, esca per favore" : again, formal form.
It's only matter of practice and get the ear (:
A punctuation question (even though DL doesn't gig you for punctuation): It seems DL always uses an exclamation point (!) after every imperative sentence. But this is a case where it's easy to see how a ! can make a difference: "How should I cut the bread?" "Dummy. Use the knife." (No big deal.) "Should I shoot him?" "Too loud. Use the knife!" (Big deal.) So, does well written Italian always require a ! or not?