I just got a little confused reading the formulation on wiki. Let $F_X$ denote the free group on the set $X$ and let the symbol $\leq$ denote "is subgroup of".
From what I know, the theorem reads: $H\leq F_X$ $\Rightarrow$ $\exists Y$: $H\cong F_Y$.
Is my formulation of this theorem also correct: $H\leq F_X$ $\Rightarrow$ $\exists Y\subseteq X$: $H=F_Y$