`For´ o `since´

for

He´s had a motorbike for two months.

They´ve lived in Portugal for ten years.

since

I´ve known my best friend since I was four.

for y since se utilizan para expresar durante cuánto tiempo ha continuado una acción hasta el presente.

* for se utiliza para hablar de un periodo de tiempo. I´ve been here for three days / a month / a year / a long time.

* since se utilize para hablar de un momento concreto en el tiempo. I´ve known how to surf since I was fifteen / since last summer / since I went to Cornwall.