Ex: He developed the sudden onset of left chest pain, which was worse with deep breathing.
Here are a few tricks of the trade:
 Take the word out of the sentence and add the to it. If the result sounds right, you've got a noun.
Ex: the onset, the pain Look for the words the, a, and an; they modify nouns.
Ex: the sudden onset Look for adjectives; they also modify nouns:
Ex: sudden onset <sudden is an adjective>
Ex: left chest pain <left chest functions as an adjective modifying pain>
Ex: deep breathing <deep modifies breathing>
 Nouns occur in two places: in subject position and in object position. Subjects tend to come before the verb, and objects tend to come after the verb or after a preposition.
Ex: developed the sudden onset
Ex: of left chest pain
Ex: with deep breathingBreathing is a verbal noun. It ends in -ing, which makes it "feel like" a verb, but it doesn't act like a verb; it functions as a noun. Nouns sit in a subject or an object position. Breathing is the object of the preposition with. Verbs can't sit there, but gerunds can. Breathing is a gerund, a verbal noun in that context.
Which is a relative pronoun.
Hope that helps. :-D