短语部分没有独立主语，则短语结构称为非谓语动词。
非谓语动词的结构：分词（分词短语）
句子的主语决定非谓语动词中的分词是用主动还是被动形式。如果是句子的主语发出动作，则使用现在分词;如果动作不是由句子的主语发出，则使用过去分词。
43.no matter how frequently _a_， the works of beethoven always attract large audiences.
a performed b performing c to be performed d being performed
perform vt. 表演，演出，演奏;
to be performed 将要被演奏; being performed 正在被演奏;
一些特殊的动词：
第一点：forget 忘记; [反义] remember 记得;
这两个词后加动词不定式表示将要进行的行为;这两个词后加动词ing形式表示已经做过的事情。
注意：动名词与现在分词形式一样，但含义不同。这两个词后加动词ing形式都是动名词。
forget to do sth. 忘记要去做某事; forget doing sth. 忘记做过某事;
remember to do sth. 记得要去做某事; remember doing sth. 记得做过某事;
62. i remember _b_ to help us if we ever got into trouble.
a once offering b him once offering c him to offer d to offer him
offer vt. 提出，提供; offer sb. sth. / offer to do sth.
him once offering 如果改成 his once offering 也正确。