I have a midterm exam today but still can't get a grasp on how to decide if given expressions are formulae or not. The three tasks are: $$1. \quad\forall y \forall x \forall yPxy $$ $$2. \quad\forall y(P \to \forall yQxy))$$ $$3. \quad\forall x(Qx)(Px \to Lx)$$
I understand the rules that atomic formulae are formulae and if A and B are formulae then $A \land B, A \lor B, \lnot A, A \to B$ are formulae and if x is a variable then $(\forall xA)$ and $(\exists xA)$ are formulae too and also if $t_1$ and $t_2$ are terms then $t_1 = t_2$ is a formula too.
Based on these rules how to I decide that given three mathematical expressions are formulae or not?