1. 单层定语从句结构：指整个句中包含一个或两个并列关系的定语从句。
例：包含一个定语从句的情况
In the late 1700s James Watt designed an efficient and commercially viable steam engine that was soon applied to a variety of industrial uses as it became cheaper to use. (TPO 26 Energy and the Industrial Revolution) 解析：该句是that引导的定语从句 固定搭配：
(be) applied to应用于
a variety of很多…... 词汇：
viable=practicable=feasible=practical=possible (adj.) 切实可行的
variety=category=group=lot (n.) 种类 翻译：十七世纪末，James Watt设计了一种高效并且可商用的蒸汽机，随着其价格降低，很快被应用于各种工业。
例：包含两个并列定语从句的情况
Without predators, the species that is the best competitor for food, shelter, nesting sites, and other environmental resources tends to dominate and exclude the species with which it competes. (TPO 27 Predator-Prey Cycles) 解析：
该句含有两个并列的定语从句，分别由that和which来引导。 词汇：
predator = eater捕食者
dominate = govern = control = predominate (v.) 控制；统治
exclude = rule out = eliminate (v.) 排除 翻译：没有捕食者时，那种在对食物、庇护所、筑巢点和其它环境资源的争夺中胜出的竞争者，趋向于成为统治者，并且使与其竞争的其它物种灭绝。
2. 多层定语从句结构：指包含两个及以上从属关系的定语从句
例：
If so, the green algae would have been subjected to environmental pressures that resulted in adaptations that enhanced their potential to give rise to land-dwelling or organisms. (TPO 25 The Evolutionary Origin of Plants) 解析：
该句主要包含了两个由that引导的定语从句（但两者并不是并列关系，而是存在等级上的分层，即多层结构）
一层：that resulted in adaptations
二层：that enhanced their potential to give rise to land-dwelling or organisms. 固定搭配：
be subjected to… 容易受到......影响
result in… 导致......
give rise to… 产生...... 词汇：
be subjected to…= be vulnerable to… = be susceptible to… 容易受到......影响
result in = give rise to = lead to = contribute to… 导致......
enhance = improve = boost = increase (v.) 提高
organism = organic matter = organic = substance (n.) 有机物；有机体 翻译：如果事实如此的话，绿藻可能曾经受到环境的压力，产生了适应机制，即提高进化出陆生植物或其他有机体的可能性。
例：
These include conducting vessels that transport water and minerals upward from the roots and that move the photosynthetic products from the leaves to the rest of the plant body and the stiffening substance lignin, which support the plant body, helping it expose maximum surface area to sunlight. (TPO 25 The Evolutionary Origin of Plants) 解析：
该句包含三个定语从句，前两个是由that引导的并列结构，共同来修饰先行词vessel, 构成*层结构。
(1) that transport water and minerals upward from the roots
(2) that move the photosynthetic products from leaves to the rest of the plant body and the stiffening substance lignin
第三个定语从句是由which引导来修饰lignin, 构成第二层结构。
(3) which support the plant body
二、同位语从句
同位语从句在托福当中也算是一个高频的结构，主要是因为ETS比较人性化，喜欢以同位语或同位语从句的形式来对专有名词或抽象名词进行解释。
例：
A major development was the discovery, again about 3000 B.C.E., that if copper, which had been known in Mesopotamia since about 3500 B.C.E., was mixed with tin, a much harder metal, bronze, would result. (TPO 26 Sumer and the First Cities of the Ancient Near East)
解析：
首先这个句子主要含有一个由that引导的同位语从句that if copper, which had been known in Mesopotamia since about 3500 B.C.E., was mixed with tin, a much harder metal, bronze, would result.
在该同位语从句中又含有一个原因状语从句if copper, which had been known in Mesopotamia since about 3500 B.C.E., was mixed with tin
在该状语从句中又含有一个定语从句which had been known in Mesopotamia since about 3500 B.C.E.
因此，如果按照句子的结构层数分析，该句应该属于三层结构的句子。
固定搭配：
be mixed with… 和…混合 词汇：
major = primary = staple = main (adj.)主要的
mix = blend = mingle (v.)混合 翻译：一个重大进展同样也发生在约公元前3000年，铜在约公元前3500年就为美索不达米亚人所熟知，如果将铜和锡混合在一起，就可以制造出一种更坚硬的金属——青铜。
例子：
Scientists have no direct evidence for recent or ongoing eruptions, but if these volcanoes were active as recently as 100 million years ago (an estimate of the time of last eruption based on the extent of impact catering on their slopes), some of them may still be at least intermittently active. (TPO25 The surface of Mars)
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