My drivers' license is valid.
A simple sentence uses a subject and a verb..
That did constitute a valid request.
The sentence is a valid combination of words. It is used to express the feelings of the person.
"At what expense?" is a valid question and so it is a valid sentence.
Complex (APEX)
This is a valid answer.
A feminist can at times be obnoxious, but still have some valid concerns.
Like this; I am an architect.
i went on an expedition
I am unsympathetic to your need.
i am a dissident
The field had boundaries.