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.
If you swallow the bait, you take something that someone offers you, or agree to do something that someone asks you to do without knowing that it is a trick or way of getting something from you.
You can't just do whatever he wants you to do. Don't swallow the bait.