since +一段時(shí)間+ ago；since +從句；It is +一段時(shí)間+ since從句；since +過(guò)去一個(gè)時(shí)間點(diǎn)(如具體的年、月、日期、鐘點(diǎn)等)。since+時(shí)間點(diǎn)，要求謂語(yǔ)動(dòng)詞用現(xiàn)在完成時(shí)，而且必須是延續(xù)性動(dòng)詞。since+一段時(shí)間+ago，要求謂語(yǔ)動(dòng)詞用現(xiàn)在完成時(shí)，而且必須是延續(xù)性動(dòng)詞。

since+從句，要求主句謂語(yǔ)動(dòng)詞用現(xiàn)在完成時(shí)，而且必須是延續(xù)性動(dòng)詞；從句謂語(yǔ)動(dòng)詞用一般過(guò)去時(shí)，而且是終止性動(dòng)詞。

It is/ has been +一段時(shí)間+since從句，意為“自從……以來(lái)已經(jīng)（多久）了”。

I have stayed in that country since 1995.

從1995年以來(lái)我就在那個(gè)國(guó)家了。

They have lived in that house since two years ago.

兩年前他們就住在那座房子里了。

Lily has stayed at home since she left school.

莉莉離開(kāi)學(xué)校之后就一直呆在家里。

It has been three months since he moved to this city.

他搬到這個(gè)城市已經(jīng)三個(gè)月了。