1But when Swetnam enrolled in graduate school at the University of Arizona's Laboratory of Tree-Ring Research, he was surprised to find a record of repeated blazes dating back hundreds of years before European colonists arrived on the continent.
主干主语：he；谓语：was surprised；宾语（不定式短语）：to find a record of repeated blazes
结构句首 when 引导时间状语从句，交代背景；主句为 he was surprised to find...，其中不定式短语作原因状语，说明惊讶的缘由；find 的宾语 a record of repeated blazes 后接现在分词短语 dating back hundreds of years 作后置定语，before 引导的时间状语从句修饰 dating back，说明火灾记录早于欧洲殖民者到来。
译文但当 Swetnam 进入亚利桑那大学树木年轮研究实验室读研究生时，他惊讶地发现了一份关于反复发生火灾的记录，其年代可追溯到欧洲殖民者抵达这片大陆之前的数百年。
考点be surprised to do sth. 中不定式表原因 · date back (to) 无被动形式，直接用现在分词作后置定语
2The NAFSN sites experienced less than a quarter of the number of fires that would have been expected without fire suppression.
主干主语：The NAFSN sites；谓语：experienced；宾语：less than a quarter of the number of fires
结构主句结构简单，难点在 fires 后的定语从句 that would have been expected without fire suppression：从句用 would have been expected 构成与过去事实相反的虚拟语气，without fire suppression 为介词短语表隐含条件，相当于 if there had been no fire suppression，意为「若没有灭火措施本可预期的火灾数量」。
译文NAFSN 各站点实际经历的火灾次数，还不到在没有灭火措施的情况下预期火灾次数的四分之一。
考点without 介词短语隐含虚拟条件，主句用 would have done · less than a quarter of the number of... 表示数量比较，注意与 a number of（许多）区分
3Yet the combined consequences of suppression and climate change have eroded humanity's ability to suppress fires, particularly those that ignite under the most dangerous weather conditions.
主干主语：the combined consequences of suppression and climate change；谓语：have eroded；宾语：humanity's ability to suppress fires
结构主句为主谓宾结构，ability 后接不定式 to suppress fires 作后置定语；句尾 particularly those that... 为补充说明成分，those 指代 fires，后接 that 引导的定语从句，限定是「在最危险天气条件下点燃的火灾」。注意主语中 combined consequences 是复数概念的核心词。
译文然而，灭火行动与气候变化叠加造成的后果，已经削弱了人类扑灭火灾的能力，尤其是那些在最危险天气条件下燃起的火灾。
考点ability to do sth. 不定式作后置定语，不用 of doing · those 作指示代词指代前文复数名词 fires，后接定语从句
4These fires were carefully planned and intentionally ignited during periods when weather kept the blazes easy to control, and helped eliminate some of fuel that had built up around important park facilities.
主干主语：These fires；谓语（并列）：were planned and ignited... and helped eliminate...；宾语：some of fuel
结构句子主干为 These fires 带三个并列谓语：were planned、ignited、helped eliminate，前两个为被动语态，第三个 helped (to) eliminate 为主动；during periods 后接 when 引导的定语从句修饰 periods，其中 keep + 宾语 + 形容词（easy to control）为复合宾语结构；fuel 后接 that 引导的定语从句，had built up 为过去完成时，表示先于主句动作积累的燃料。
译文这些火经过精心策划，在天气条件使火势易于控制的时段被有意点燃，帮助清除了公园重要设施周围积累的部分可燃物。
考点并列谓语中被动与主动混用，需理清逻辑主语 · keep + 宾语 + 形容词补语结构；过去完成时 had built up 表「过去的过去」