F. Le 了is used for asking and answering questions about how long one has not done something or, in other words, time elapsed since one last did something.
Sentence Pattern: subject + (adverb) + duration + méi没(or méi yǒu没有) + verb + (object) + le了
It is important to note that this pattern places the duration before the verb and uses méi没 and le了 in the same sentence.
6.1 Wǒ yǐjīng liǎng tiān méi(yǒu) chī fàn le.
I have not eaten for two days.
6.2 Tā yǐjīng sān nián méi xué Zhōngwén le.
He has not studied Chinese for three years.