One day a friend of mine from the East wrote me a letter.

Many sketches are in existence of peasants seated by the roadside and men and women at work in the fields.

An element is  a substance that cannot be broken down into simpler substance by ordinary chemical means.

老师，介词短语，非谓语短语（多个非谓语短语），在句子中作定语或状语时，如何区别？通常通过中文的意思才知道吗，又或句子解释起来通顺就得吗?

像这些不知哪个是修饰语，有些还是分隔的定语是吗？