Yes, in CTL, temporal operators and path quantifiers always occur in pairs, i.e. EX, EG, EF, E[. SU .], E[. WU .],E[. SB .], E[. WB .], AX, AG, AF, A[. SU .], A[. WU .],A[. SB .], A[. WB .]. That is the restriction for a CTL* formula to become a CTL formula.
Concerning LTL, we consider formulas of the form A phi where phi does not contain any path quantifier. Hence, AG(a ∧ ¬b) is a LTL formula since there is no path quantifier in a ∧ ¬b.
However, AG(a ∧ ¬b) is not valid: Validity means that the formula should hold on every Kripke structure.
There is not much more to say about the syntactical restrictions of CTL and LTL. That is the simple part. It is much more difficult to check whether a given formula could be rewritten to an equivalent CTL or LTL formula.