回複：confused!

Usually, a foreigner can stay in US while his visa is vaild. for example, if B visa says it expires in 6 month, the person must leave U.S or change to other visa before it expires.

For H and F, it is a little different. Although the visa syas it expires in 6 months, you can stay much longer than this. Usually we say you are on H or F status. For example, when you entered US with F-1 visa, the visa expires in 6 months, but U.S Custom officer will write DOS on your passport, which means "duration of study". This means you are on F-1 status as long as you are still in the same study programm indicated by your I-20.

If you go back to China during your F-1 status, you have to turn in your I-94, and your F-1 status ends. If your F-1 visa is still valid (within 6 months of issuing date), you dont need a new visa to enter U.S.. Otherwise, you have to reapply for F-1 visa to enter U.S.

For H, it is the same. If you leave U.S, your H status ends. If you want to come back, you have to reapply H-1 visa. (you dont have to if the previous H-1 visa is still valid, or within 6 months of issuing date).

If you dont want to re-apply H1 visa, you can use AP to enter, and you still can go back to the same company to work. But your status is not H anymore. your status become i-485 pending.