Réponse :
A 1.- He has been working since 2000.
2.- Last night David and Laura were in my house.
3.- Before going to my father, allow me to say.
B 1.- The younger you are, (stronger, strong, the stronger) you are.
2.- Billy hasn't called anybody. (since, For, Ago) the meeting.
Explications :
Since : évènement précis (date, moment...)
For : durée (en mois, jours, années...)
Ago : équivalent de "il y a (+durée)