CORRECT: I can't see the road ahead. There is no more food left. If I'd only known that earlier. / If I'd only known that sooner. (A slight difference in meaning between the the two. 'Earlier' implies earlier in the day (by the hour). 'Sooner' could mean the same thing or it could refer to before you did some action. For example, "I wish I'd known that sooner" = "I wish you'd told me you'd already eaten before I had cooked dinner.") I'm having trouble with paying my debts. I knew him a long time before we met. (Ambiguous. This could mean either 'before he and I met' OR 'before you and I met.) I knew him a long time before I knew you. (better)