用作表語的不定式符號to可以省略嗎

當不定式用作表語時，其中的to原則上不宜省略。如：

The important thing is to save lives. 救人要緊。

The problem is to find the right house. 問題是要找到合適的房子。

His plan is to keep the affair secret. 他的計劃是將這件事保密。

The idea of the game is to hit the ball over the net. 這個游戲的意思就是要將球打過網去。

但有，若主語部分含有動詞do（及其變化形式），則用作表語的不定式可以省略to。如：

The only thing to do now is (to) go on. 前進是現在惟一的出路。

What I’ll do is (to) tell her the truth. 我要做的就是告訴她真相。

All I did was (to) press this button. 我所做的只是按這個電鈕。

What you have to do is (to) fill in the questionnaire. 你要做的是把問卷填好。