帮助他的是他的三个姐姐。
It was his three sisters who/that helped them.
他们寻找的是李华。
It was Li Hua that/whom they were looking for.
注意:is/was 不随强调内容的单复数而变化。
妙招:去掉it is/was…that/whom/who 之后原句子完整,为强调句型。