1. already用于肯定句中,表示已经完成的动作,如"I have already finished the work." 同时,它常与yet进行对比,形成否定句"I haven't finished the work yet"和一般疑问句"Have you finished the work yet?"
2. yet常用于否定句和一般疑问句句末,如"I didn't receive a letter from him yet." 或者询问是否已完成的动作,如"Have you eaten yet?"
3. just用于陈述句,表示刚刚完成的动作,如"I have just received a letter."
4. never表示从未,用于否定句中,如"I've never been to Beijing." 或者询问过去的经历,如"Have you ever been to Beijing?"
5. ever在疑问句中表示曾经,如"Have you ever been to Beijing?"
6. before用于肯定句,表示动作在过去的某个时间点之前发生,如"He has never been to Beijing before."
7. since则与延续性动词连用,根据时态不同,可以表示从过去某个时间点到现在(现在完成时)或动作开始以来(现在完成时或一般过去时),例如"It's been three years since I worked in this company" 或 "It's been three years since I have worked in this company."