In the Benoni, a common move in the position is e4-e5 and then Ng1-f3. Afterwards, I have worries about my opponent playing e4-e5 and creating a passed pawn, but up to now, I haven't met with this situation yet. How can I defend against e4-e5 properly?
This is the crux of the Modern Benoni, right?
Do you know why you're playing the Benoni in the first place? You're trying to post a knight on e5. That's why that square is most critical; White is trying to run over your backwardness at d6 (the reason 6. e4, 7. f4, and 8. Bb5+ is White's most successful), and Black is trying to win the square himself because it's such a great outpost.
If Black had a fully adequate remedy for this, the Benoni would be much more popular. This is the price you pay for this splendidly unbalanced position, your open e-file, your queenside pawn majority: White might crush you in the center. The best you can do is point your pieces at it; I mean when Bb5+ comes, interposing with f6-knight stands a better shot because then the g7-bishop is directly on e5.
You might try Dzindzichashvili's peculiar 1. d4 g6 2. c4 Bg7 3. Nc3 c5 4. d5 Bxc3+ 5. bxc3 f5, a weird hybrid of Benoni, Dutch, Nimzo-Indian, aimed at gaining enough control over e4 so White can't so easily achieve e2-e4-e5, etc.
Good luck. Study Tal games.