One uses hace + expression of time + a verb in the present tense to indicate an action which began in past time and continues in the present time.

Hace un mes que estoy trabajando.
I have been working for a month.

Hace unos tres años que estudio español.
I have been studying Spanish for about three years.

The hace clause may be used either before or after the main verb; if used afterwards, desde is inserted before hace.

Estudio español desde hace dos años.
I have been studying Spanish for two years.

Están divorciados desde hace un año.
They have been divorced for one year.

Let's look at some some slightly different ways to use the verb hace to discuss the passage of time.

¿Hace cuánto tiempo tiene el dolor?
How long have you had the pain?

Hace quince minutos.
It's been fifteen minutes.

Hace una hora.
One hour ago.

Preterite Tense

The same construction is used to indicate an action which began and ended at an earlier time.

Eso pasó hace quince minutos.
That happened 15 minutes ago.

El taxi se fue hace una hora.
The taxi cab left one hour ago.

Imperfect Tense

The same construction, but using the imperfect hacía + expression of time + a verb in the imperfect tense is used to indicate an action which is continuing on in past time but which started at an earlier time.

Hacía una hora que estudiaban.
They had been studying for an hour.

¿Hacia cuánto tiempo que trabajaba usted?
How long had you been working?

As before, hacía may be used either before or after the main verb; if used afterwards, desde is inserted before hacía.

Quería rodar esta película desde hacía un año.
He wanted to shoot this movie for a year.

Buscaba su teléfono desde hacía una hora.
He was looking for his phone for an hour.