To say how long something has been happening or how long ago something happened you can use the following formula:
Hace + amount of time + que + conjugated verb
or
Conjugated verb + desde hace + amount of time
The formula always stays the same, but the meaning of the sentence is changed by using different verb tenses.
Present Indicative Tense
Use the present indicative tense to say how long you have been doing something or how long something has been happening.
Hace una hora que leo mi libro. / Leo mi libre desde hace una hora. → I have been reading my book for an hour.
Hace dos años que estudio. / Estudio desde hace dos años. → I have been studying for two years.
¿Cuánto tiempo hace que comes? / ¿Desde cuándo comes? → How long have you been eating?
Preterit Indicative Tense
Use the preterit tense to say how long ago something happened.
Hace una hora que leí mi libro. / Leí mi libre desde hace una hora. → I read my book an hour ago.
Hace dos años que estudié . / Desde hace dos años estudié . → I studied two years ago.
¿Cuánto tiempo hace que comiste? / ¿Desde cuándo comiste? →How long ago did you eat?
Imperfect Indicative Tense
Use the imperfect indicative tense to indicate how long something had been happening.
Hace una hora que leía mi libro. / Leía mi libro desde hace una hora. → I had been reading my book for an hour.
Hace dos años que estudiaba . / Desde hace dos años estudiaba . → I had been studying for two years.
¿Cuánto tiempo hace que comías? / ¿Desde cuándo comías? →How long had you been eating?
*Notice that the only difference between the sample sentences is that the tense of the verb changes and nothing else.
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