"This is the story of a group of extraordinary men who I was lucky enough to serve alongside as a SEAL from 1998 to 2012."See the problem? Mark should have written that sentence this way: "This is the story of a group of extraordinary men alongside whom I was lucky enough to serve as a SEAL from 1998 to 2012."
There's actually a pretty simple rule that tells you when to use "who" and when to use "whom." Who is always a subject and whom is always an object. So, when you're wondering whether to use who or whom (or whoever or whomever), substitute the word he/she/they (subjective) and him/her/them (objective) and see which one sounds right.
Confused? Here's what I mean:
- "Who/Whom shall I send?" (Isaiah 6:8a)
- I shall send him (objective). Therefore, "Whom shall I send?" is correct.
- "Who/Whom will go for us?" (Isaiah 6:8b)
- He will go for us (subjective). Therefore "Who will go for us?" is correct.
"Who (subject) has believed our message and to whom (object) has the arm of the Lord been revealed?"