Sub-sentence

Adv.
* time: when, once, as, while
when > once > as

* goal: so that, in order that

* result: so / such ... that, so (that)

* reason: because, as, since
because > as > since

* RangBu: thought, although, even though, even if, whenever, where ever, however, while, however much

Noun.
* who, which, what, when, where, how, why, that, if, whether
* order of sub. verb.
* whatever, whoever
* whether
* that (TongWeiYu)

DingYu
* 关系代词的数和人称要和先行词一致, 注意主格，宾格
* 关系副词的先行词总是时间，地点或理由的名词
* that, whom may omit or not
* that vs. which
* 当先行词为介词宾语，一般要介词前置
* n./pron. + of + which / whom
* which 先行词绝对不能是句子
* 先行词靠近原则

* omission
* when, while, if, though, until, once, whether, unless, no matter what,
引导的从句中，谓语动词为be，而主语与主句的主语相同，从句的主语和动词be常省略

* A1 is to A2 what A3 is to A4.
* A logarithm is what is known in algebra as an exponent.
* what is now known as A
* while + doing