What is the most reactive atom on ethylimine when it comes to nucleophilic attack or reaction with a base?

Ethylimine doesn't really have more than one good resonance structure, but looking at the ones it has can suggest which atom is most reactive on it.

We don't have any lone pairs from the carbons that can withdraw onto nitrogen for the resonance structure, but we do have a #pi# bond on the #"C"="N"# connection. That can donate into nitrogen's empty #sp^2# hybrid orbital.

We should know that secondary carbocations aren't extremely stable, but we should also keep in mind that nitrogen is only about #0.5# more electronegative than carbon (unlike oxygen, whose electronegativity is about #1.0# higher).

Nitrogen is not quite as good of an electron-withdrawing group as oxygen, so imines aren't as reactive as ketones are.

So while that electropositive carbon is reactive, it tends to need a bit of encouragement before it becomes receptive enough to a nucleophilic attack, like when using trace acid.

On a related topic, we could add some trace acid, like dilute #"HCl"# for instance, to facilitate its tautomerization into an enamine.

Tautomerization isn't the same thing as resonance, but it could be considered a type of isomerization catalyzed by acid.

Both the neutral imine #"H"_3"C"-("HC"="NH")# and the new, neutral enamine (#"H"_2"C"="CH"-"NH"_2#) have the same molecular formula, but a different structure.

Both are #"C"_2"H"_5"N"#. But it's not resonance.