Keep behind phrasal verb
A noun or pronoun can be used in the first meaning between "keep" and "behind". In the second meaning, the phrasal verb is usually used in the passive form.
If a person "keeps something behind something else/someone", he/she literally holds something behind.
He keeps his toys behind the shelf in order to hide them from his younger brother.
If a person "is kept behind", he/she fails an exam, so they have to repeat a year/grade.
You know what? He was kept behind last year because of his regular playing hooky.
The verb "keep" should be conjugated according to its tense.
Difficult to choose between two alternatives.
I am always caught between two stools when choosing what to buy at a bookstore.