再长的句子，主干也是主谓宾或主系表这几个成分，甚至只有主谓，也就是说，句子长只是因为修饰语多，越长的句子，修饰语越多。分析长句，就是分清哪是主干，哪是修饰语，理清它们之间的关系，这个过程，需要语法。对于作者来说，语法是构建句子的工具，对于读者来说，语法是剖析句子的工具。
例如这个句子：
A Canadian is a person wearing English tweeds, a Hong Kong shirt and Spanish shoes, who sips Brazilian coffee sweetened with Philippine sugar from a Bavarian cup while nibbling on Swiss cheese, sitting at a Danish desk over a Persian rug, after coming home in a German car from an Italian movie…and writes to a Member of Parliament with a Japanese ballpoint pen on French paper, demanding that something be done about foreigners taking away our Canadian jobs.
其实主干就是开头的四个单词“A Canadian is a person”，其他的全是person的修饰语，全家大意是“加拿大人就是一个……的人”。
