▲你的老师说的没有错。下面两句第一句对，第二句错：

正：If he should come, tell me at once. 万一他来，马上告诉我。

误：If he were ill, tell me at once. 万一他病了，马上告诉我。

【分析】

◎根据英语语法，只有在条件从句中用“should + 动词原形”，主句才可用祈使句。又如：

(1) Write to me if you should need help.万一需要我帮助，写信给我。

(2) If it should rain tomorrow, don’t expect me. 万一明天下雨，就不要等我了。

(3) If you should write to her, send her my love. 如果你给她写信，请代我向她致意。

(4) If he should call, tell him I’ll ring back. 如果他来电话，告诉他我将给他回电话。

(5) If you should change your mind, do let me know. 万一你要改变主意，一定要告诉我。

(6) Let me know if you should hear some more news. 万一你听到更多消息你要通知我。

◎事实上，当条件从句的谓语动词为“should+动词原形”时，主句谓语动词既可以用虚拟语气，也可以用祈使语气，还可以用陈述语气。如：

(1) If he should lose, I shall (should) feel sorry. 万一他输了，我会感到遗憾。

(2) If we should fail in this, we are (would be) ruined. 如果这件事失败，我们就毁了。

(3) If he should call, tell (you can tell) him I’m not home. 他若打电话来，你就告诉他我不在家。