You are certain that you hold the worst hand and are also facing a bet.
You don't have enough chance of improving to the best hand compared to the pot odds you get.
You're unsure whether you're ahead or behind, but you'd have to call a big bet now or maybe later in the hand to find out.
You are getting odds to call but you aren't closing the action and it is very likely that someone behind you will make it too expensive to continue.
Situations in which folding would be incorrect
You can check instead :-).
you know you have the best hand (obviously).
You know you probably have the worst hand but you are getting the correct odds to call with your draw anyway.
You know you are behind, but you are also pretty sure that a bluff will win you the pot.
Agree with these answer,
btw poker is not just theory, you better play and make your own decision on when to fold or call.
Getting Mathematical
The information you have about your opponent's hand is that he limped from early position, then called a small raise, and when the flop came out he led into you for two-thirds the pot on a 7♠ J♣ Q♦ board.
You can assign him a range of something like TT-77, KQs, Q9s+, J9s+, T8s+, ATo, KQo, QTo+, JTo, T9o - which is a pretty wide range. Against it you're a 40% dog.because the bulk of that range is weak
he will fold to a raise a high percentage of the time. What the exact percent is is impossible to determine, although it's safe to say that he will be folding enough times to make raising a more profitable play on your part than folding.
Nice post, I agree, you can try to pick them on a hand, and usually you can get somewhat close. Another huge part of poker though is being able to read people so you can calculate what they have, or if they are bluffing.