implication versus conjunction correctness in FOL? I've just started learning FOL and I'm really confused about whether to use conjunction or implications. For example, if I want to represent
some students who answer the easiest question do not answer the most difficult

I came up with several solutions that seem equivalent to me.
1) ∃x. (student(x) Λ solve(x, easy) Λ ¬solve(x, hard))
2) ∃x. (student(x) -> (solve(x, easy) Λ ¬solve(x, hard)))
3) ∃x. ((student(x) Λ solve(x, easy)) -> ¬solve(x, hard)))
Can anyone explain which is correct and why the others are wrong?

It depends on the structure on which you evaluate your formulae. For simplicity I would introduce $3$ predicates $\mathsf{student}, \mathsf{solve\_easy}, \mathsf{solve\_hard}$. (The parameterized solve works but I think it is a little confusing)
If the universe of your structures contains both students and non-students (which I assume because you introduced the predicate $\mathsf{student}$) then:


*

*Formulae $1$ is correct. It states the existence of a student who answers easy questions but not difficult ones.

*Formulae $2$ is not correct. Assume that students solve no questions at all and there is at least one non-student. By the definition of implication the non-student satisfies $\mathsf{student}(x) \rightarrow \varphi(x)$ for any $\varphi$. Hence, the formulae is satisfied but it shouldn't be.

*Formulae $3$ is not correct. Assume again that students do not solve any questions and that there is a non-student.


If all elements in the universe are students all $3$ formulae are correct.
