Q: Past simple vs present perfect inquiry When I meet someone and I doubt that I know him. Should I say : 
1- I think we have met before.
2- I think we met before. 

A: "I think we've met before" actually means that you are more certain than doubtful, so it's not a good choice if you're actually not sure you know the other person. "I think we met before" has the wrong tense for what you're trying to say. 
Here's one option - if someone looks familiar to you, but you're not sure whether you've actually met that person, you could ask: "Have we met before? You look familiar."
Alternatively, if you've just met someone, and as you're talking, you realize you may have met each other already, you might say: "I think we might/may have met before." The "might" or "may" here expresses the fact that you're not sure.
