Explanation:
First, a quick recap:
Nonessential elements of a sentence are words, set of words (phrases) or clauses that can be left out of the sentence without altering its meaning, unlike the essential elements which have opposite feature.
Introductory phrase is a set of words that provides setting for a sentence.
So, for example, we could say:
A: William Shakespeare, famous writter, wrote many plays.
B: The dog that I found by the river hates swimming.
C: On monday morning, luckily, I found my keys.
D: I visited Paris, the capital of France.
Note that nonessential elements are always separated by commas.
