Charles Ellicott Commentary Exodus 40:3

"And thou shalt put therein the ark of the testimony, and thou shalt screen the ark with the veil." — Exodus 40:3 (ASV)

Thou shalt put therein the ark of the testimony. — The most precious of the objects which the tabernacle was to contain was to be placed in it first. Then immediately it was to be covered up with the veil.