I believe either of those can be correct. " if I had gone to view a specific problem (say the damages from a hurricane) on one particular occasion. I would tend to use, "From what I have seen," if it was a problem that I'd been reviewing over some period of time.
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Picnicsay I want to say: "From what I saw this is not easy to resolve"From what I've seen is better -- you might say it's more correct -- but you could get away with the first one in casual conversation.
is this form correct or should I say "From what I have seen this is not easy to resolve"