When do I use the "imperfetto" ?

The imperfetto is a tense used to express the duration of a past action without referring to its beginning, end or purpose. It is often used to describe:

Recurring actions:

Due mesi fa, andavo in palestra quattro volte alla settimana.

To describe circumstances:

Era una bella giornata.

Describe ongoing actions interrupted by other actions expressed with passato prossimo (present perfect):

Leggevo un articolo, quando è arrivato Mauro.