Which is correct?
My vision widened that day, and I began to see the trap that lay ahead for most people.
My vision widened that day, and I began to see the trap that lie ahead for most people.
Or maybe both are incorrect and some future tense of lie must be used?
I am confused because someone is talking about his past and others’ future that may or may not have faced the trap by the present. The trap the author talks about is kind of a fact of life.
Assuming you change 'lie' to 'lies' in the second variant, both sentences would be correct, but have a slightly different emphasis.
You need to change 'lie' to 'lies' because 'the trap' is third person singular and requires an 's' in present indicative. (If you google: "Lie verb table" you will get lots of hits on the grammatic form. I'm linking to one here).
In the first, the past tense is used consistently. The account is all in the past: began to see the trap that lay ahead. (From the perspective of the writer both events are in the past. But at the point that he began to see the trap, the trap was in the future; thus 'ahead')
In the second the indication is that at that (past) time he began to see a trap that still or generally lies ahead for most people.
I realise from your comment that you didn't quite understand, so I'll see if I can make it clearer. Imagine I'm writing now as an adult about an event from childhood. I was learning to swim.
I had had three swimming lessons and decided I could swim, but I did not know about the trap that lay ahead. I swam out into the ocean as far as I possibly could, but there were strong undercurrents and I could not swim back. In the end I was rescued by a lifeguard.
(In the example, the trap does not lie ahead from my present day perspective. But it lay ahead before I swam out into the ocean -- thus 'lay'.)
I'm still an adult and writing now about something else from childhood.
I was 4 years old and assumed I would live forever. I did not know then about the trap (death) that lies ahead for all of us mortal creatures.
(This 'trap' lies ahead from my present day perpective.)
I hope that helps.
Lay can either be used as a transitive verb (one that requires a direct object) or as the past tense of lie. If you’re speaking in the present tense in your example, then you should use lies ahead, since there is no direct object. And make sure to say “lies ahead,” as “trap” is singular, not plural.
Then again, if you are speaking in the past tense, then you would say that the trap “lay ahead,” even though it sounds like a present usage—the past tense for the transitive verb lay would be laid.
