already 是己经的意思,eg：He has already eat breakfast .
yet用在句末,eg:He has not had any milk ,yet.
never从不,此为频率副词,eg：He has never eat egg.
ever 曾经,常用于现在完成时的疑问句中,:egHave you ever had French food?
for后跟一段时间,eg:I have already eat breakfast for yesterday .
since后跟时间点,eg:I have eat breakfast since half past seven .