A vertex is either an internal vertex or a leaf. Since the number of all vertices is $n$ while the number of internal nodes is $i$, the number of leaves, $l$ is $n-i$.

Every vertex is identified as a child of an internal node except the root. Since there are $i$ internal node, each of them having $m$ children, the number of vertices is $m\cdot i + 1$.

A vertex is either an internal vertex or a leaf. Since the number of all vertices is $n$ while the number of internal nodes is $i$, the number of leaves, $l$ is $n-i$.

Every vertex is identified as a child of an internal node except the root. Since there are $i$ internal node, each of them having $m$ children, the number of vertices, $n$ is $m\cdot i + 1$.

Since $i= n-l$, we also have $n = m\cdot (n-l) + 1$. Or $$(m-1)n = m\cdot l - 1,$$ which is the relation between the number of vertices and the number of leaves in a full $m$-ary tree.