Passato Prossimo vs. imperfetto (parte 1ª)

When to use Passato Prossimo and when to use Imperfetto is a common challenge for English-speaking Italian learners.
While many learners were taught that imperfetto is used for expressing  "used to [do]" and "was [doing]", and Passato Prossimo for past actions like "have [done]", English also uses simple past (did, went, slept...) for many past events, while Italian makes a distinction.

One quick way to make a distinction is to pay attention to time expressions:

... often used with Passato Prossimo

stamattina
ieri 
l’altro ieri
ieri sera 
l’altro giorno
la settimana scorsa
il mese scorso
l’anno scorso 
appena    

this morning
yesterday
day before yesterday
last night
the other day
last week             
last month
last year
just 

... often used with Imperfetto

sempre
spesso
di solito
mentre
ogni giorno
da piccolo/a
da giovane
a volte
ogni tanto

always
often
usually
while
every day
when I was little 
when I was young
at times
once in a while

Passato Prossimo
esempi:

Ieri sono andato al negozio.
Ho parlato con mia Nonna stamattina.
Ho vissuto a Londra per due anni.
Abbiamo appena mangiato.

I went to the store yesterday.
I spoke with my grandmother this morning.
I lived in London for two years.
​We've just eaten.

Imperfetto
esempi:

Da bambina avevo i capelli lisci.
​Andavo spesso al cinema.
Da piccoli, ogni estate andavamo in Spagna
​
​Mentre guardavo la TV, mia mamma preparava la cena.
​

When I was a child I had straight hair.
I often used to go to the movies.
​When we were little we used to go to Spain every summer.
While I was watching TV, my mom prepared dinner.
​

NOTA BENE 
If the time period is indefinite but the action occurred a specific number of times, the passato prossimo is used:

Sono andata a trovare i nonni centinaia di volte.

Da piccola ho guardato una volta un film horror che mi ha davvero spaventato.

I've visited my grandparents hundreds of times. 

When I was little I once watched a horror film that really scared me.

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Continueremo la prossima volta con la seconda parte.