Ah yes, it's been said before -- context.
If, for example, there is a question about who killed the snake. Two people are having a conversation about it.
"I suppose Carla could have killed the snake."
"No, John said that he killed the snake."
"Did he kill the snake after Carla saw it?"
"No, he had killed the snake before she got home from work."
Take note, however, because of strange contractions many native speakers can be unaware of certain grammatical constructions. For example, "I'd a done it if I'd it woulda been necessary." translates to "I would have done it if it would have been necessary."
Sometimes the "have" part is pronounced "of" and can lead to incorrect writing -- I would of seen him do it if ...."