exercice 1: "since january 1st"
"since yesterday"
"for a seconde"
"since last night"
"since half past two"
"for a week"
"for half an hour"
"since the beginning of the year"
"since they were born"
"for a year"
"since the last holidays"
"for two centuries"
"since the Middle Ages"
"since I arrives"
"for a long time"
exercice 2:
a) for
b) for
c) since
d) since
