其实这关乎延续性动词的问题.一般多用于现在完成时.become、buy等都是短暂性动词
延续性动词表示能够延续的动作,如：learn,work,stand,lie,know,walk,keep,have,wait,watch,sing,read,sleep,live,stay等.延续性动词可以与表示时间段的状语连用.
表示时间段的短语有:⑴ for+一段时间,如 ：for 2 years; ⑵ since从句,如 since he came here; since+过去时间点名词,如 since last year,since 5 days ago.