I'm just taking a guess here..
But language techniques are actual techniques or whatever they use (lol I'm bad at explaining) .. like metaphors, hyperbole, juxtaposition.
whereas language structure is the form its written in.. such as 3 lines a stanza, the rhythm, how the composer pieces it together.
My answer sounds terrible haha, I'm sure someone else can explain it better.
as for language feature.. Erm I'm not sure.. is it like facial expressions and other stuff like that?