Hacer, Llevar with Time Expressions  
  The verbs hacer and llevar, along with time expressions and an event described in the same sentence, are used to expressed the amount of time elapsed between two points established by the context. Tenses used in the Spanish sentences generally differ from their English counterparts.  
  In the structures presented here, hacer is always used impersonally, occurring only in third-person singular, and only in present indicative (hace) and imperfect indicative (hacía).  
  Time elapsed since a completed past event occurred (...ago...)  
  • Event in past + hace or Hace + event in past + que
    • Mi padre salió para Chicago hace dos horas. / Hace dos horas que mi padre salió para Chicago.
      • My father left for Chicago two hours ago.
    • El senador murió hace dos meses. / Hace dos meses que el senador murió.
      • The senator died two months ago.
  Time elapsed since an event (still in progress) began  
  • Hace + time expression + que + verb in present
    • Hace dos años que ellas estudian francés.
      • They have been studying French for two years.
    • Hace tres semanas que no los vemos.
      • We haven't seen them for three weeks.
  Time elapsed between event in remote past and more recent point in past  
  • Hacía + time expression + que + verb in imperfect indicative
    • Cuando dejé de fumar en 1998, hacía veinte años que fumaba.
      • When I quit smoking in 1990, I had been smoking for twenty years.
    • Vimos a César ayer en el centro. Hacía dos años que no lo veíamos.
      • We saw César downtown yesterday. We hadn't seen him for for years.
  Unlike hacer, llevar is used in all persons when used to describe time elapsed between events. There are some other structural differences as well, as the examples below show. Too, use of llevar for this purpose is generally restricted to events of longer durantion.  
  Time during which an event (still in progress) has been occurring  
  • Llevar (conjugated in present) + time expression + ____ndo... (present participle)
    • Llevo tres meses trabajando en esta tienda.
      • I've been working in this store for three months.
    • ¿Cuánto tiempo lleva Ud. dando clases de física?
      • How long have you been teaching physics classes?
  Time during which an event has not been occurring  
  • Llevar (conjugated in present) + time expression + sin + infinitive
    • Llevamos mucho tiempo sin leer ese periódico.
      • We haven't read that newspaper for a long time.
    • Mi hermano lleva diez años sin fumar.
      • My brother hasn't smoked for ten years.
  Past equivalent of above llevar sentences  
  In the sentences below, time duration between two points in past are described. The same structures as above are used, but with conjugated verbs appearing in the imperfect indicative.  
  • LLevaba tres meses trabajando en esa tienda cuando me aumentaron el sueldo.
    • I'd been working in that store for three months when they raised my pay.
  • ¿Cuánto tiempo llevaba Ud. dando clases de física cuando decidió jubilarse?
    • How long had you been teaching physics classes when you decided to retire?
  • Vimos una copia de ese periódico en la librería ayer. Llevábamos tres meses sin leerlo.
    • We saw a copy of that newspaper in the bookstore yesterday. We hadn't read it for three months.
  • Mi hermano fumó hoy un cigarrillo que su amigo le ofreció. Llevaba diez años sin fumar.
    • Today my brother smoked a cigarette that a friend offered him. He hadn't smoked for ten years.