Bonjour,

ago = il y a

since = ( point de départ ) depuis

for = ( durée ) depuis

Réponse :

Grandma was in Paris two weeks ago.

He has been on the phone since two o’clock.

Attention : since two o'clock.

(=> depuis deux heures ( l'heure marquée sur l'horloge/point de départ ))

                  for two hours.

(=> depuis deux heures / durée )

He has flown this plane for ages.

They have been married for ten years.

He started delivering letters twenty years ago.

It has been raining since Monday.

He sold a new house two days ago.

He has been recycling since January.

They have been in love for three years.

;)