Is a finite field extension of a imperfect field imperfect Let $K$ be a imperfect field. Let $L/K$ be a finite field extension. Is $L$ imperfect? Suppose that $L/K$ is separable. Is $L$ imperfect? Suppose that $L/K$ is Galois. Is $L$ imperfect? I'm looking for examples, counterexamples, explanations, and proofs of course. **Note:** The converse holds. If $K$ is perfect, then any finite field extension of $K$ is perfect.

Yes, if $K$ is imperfect then any finite extension $L$ of $K$ is also imperfect.
Indeed if $char.K=p$ and $a\in K$ has no $p$-th root in $K$, then any perfect field $P$ containing $K$ must contain all $a^{p^{-n}} \: (n\geq 1)$ and, since $[K(a^{p^{-n}}):K]=p^n$, the field $P$ must be of infinite dimension over $K$ (since its dimension is $\geq p^n$ for all $n$).
The fact that $L$ is separable or Galois over $K$ will not help: I'm sorry to say that $L$ will still be imperfect (but aren't we all...)