Vector Subspace of Hausdorff Topological Vector Space is Hausdorff Topological Vector Space
Theorem
Let $K$ be a topological field.
Let $\struct {\struct {X, +_X, \circ_X}_K, \tau_X}$ be a Hausdorff topological vector space over $K$.
Let $Y$ be a vector subspace of $X$.
Let $\tau_Y$ be the subspace topology on $Y$ induced by $\tau_X$.
Let $+_Y : Y \times Y \to Y$ be the restriction of $+_X$ to $Y \times Y$.
Let $\circ_Y : K \times Y \to Y$ be the restriction of $\circ_X$ to $K \times Y$.
Then $\struct {\struct {Y, +_Y, \circ_Y}_K, \tau_Y}$ is a Hausdorff topological vector space.
Proof
From Vector Subspace of Topological Vector Space is Topological Vector Space, $\struct {\struct {Y, +_Y, \circ_Y}_K, \tau_Y}$ is a topological vector space.
Since $\struct {\struct {X, +_X, \circ_X}_K, \tau_X}$ is a Hausdorff topological vector space, it is in particular Hausdorff.
By Subspace of Hausdorff Space is Hausdorff, since $\struct {\struct {Y, +_Y, \circ_Y}_K, \tau_Y}$ is a topological subspace of $\struct {\struct {X, +_X, \circ_X}_K, \tau_X}$, we have that $\struct {\struct {Y, +_Y, \circ_Y}_K, \tau_Y}$ is Hausdorff.
So $\struct {\struct {Y, +_Y, \circ_Y}_K, \tau_Y}$ is a Hausdorff topological vector space.
$\blacksquare$