1. 先行词是 April 29,2011

因为后面的引导词是when，是引导时间的，所以它的先行词是表示时间的April 29,2011

when就相当于这个时间， Prince William and Kate Middleton walked into the palace hall of wedding on April 29,2011

句子也可以表达成 It was April 29,2011 on which Prince William and Kate Middleton walked into the palace hall of wedding.

因为when= on/in/ at which

这里的on which= on April 29,2011

2. 先行词是energy

which就相当于energy

句子陈述的表述是 we may return to the energy in the near future

所以就to which we may return in the near future

3. 先行词是play

which代替了play

句子还原是my student acted in the play at the New Year's party

一般情况下，先行词都是位于引导词的前面，如这里的引导词when, to which, in which等，都是它前面的那个名词。

而且先行词都是可以被这些引导词代替的，如in the play= in which

we may return to the energy=we may return to which= to which we may return

(通常介词要位于which的前面，如on which, in which, at which)

4. to which

因为是I paid a visit to sp

pay a visit to sp .

现在的sp. 被which代替， 变成 I pay a visit to which. 然后介词要提前，变成 to which i pay a visit