How do i fix the sentence: Before putting the vase on the shelf, he dusted it.
Never start a sentence with Before...He dusted the vase before putting it on the shelf.
You could write the sentence in this manner; before placing the vase on the shelf, he dusted it. You could also write it as; he dusted the vase before placing or putting it on the shelf.
He dusted the vase then put it on the shelf.
or..drop the comma..
He gently dusted the lamp, and then thereafter, he placed it on the shelf. :D