So, I found this explanation of when and when not to set off adverbs with commas.
Here is an example in which 'therefore' serves as more of an aside or a pause:
All of the test animals, therefore, were re-examined.
In this case, 'therefore' is bounded by commas to separate it from the rest of the sentence and to provide a pause for the reader. In this example, if 'therefore' were moved and placed within the verb 'were re-examined', it would be treated as an essential (restrictive) adverb and would not require commas:
All of the test animals were therefore re-examined.
But, I'm confused as to why 'therefore' suddenly becomes restrictive when inserted into the verb, 'were re-examined'. It's still the same sentence in meaning, right?
My guess as to the reason why was that we don't want to use a comma or commas to divide the words that comprise a compound verb. But, then I thought of an example where it would be fine to divide the words of a compound verb! Here it is:
If Star Wars were, for example, deemed inappropriate for children, Disney would make relatively less money.
So, my question is why does 'therefore' suddenly become restrictive when placed between 'were' and 're-examined"?