If you are teaching your intermediate Spanish students present and imperfect time expressions, I have a writing activity just for your classes! This activity provides examples of how to use the two different time constructions in the Spanish language, followed by many practice exercises and translations.
The two formulas used in this activity are:
Hace + time + que + present tense (to say how long something has been happening)
Hacía + time + que + imperfect tense (to say how long something had been happening)