Why can hydrogen be placed in either group 1 or 7 on the periodic table?

Hydrogen can be in both group 1 and group 7 due to it's valence shell. Hydrogen only needs 2 (or 0) electrons to be stable.

Because it has 1 electron, it can either accept one (like a group 7 element) or give one away (like a group 1 element).

It can act like either group.