Change the g to y, and spell things phonetically and you'll get tre-yard ie tree yard, which is exactly where this word comes (from Proto-Germanic trewą and gardaz, something like trewgardaz, I guess?)
A "gård" can also be the yard of a block of apartments. A "trädgård" is supposed to be green, but if you are unlucky a "gård" can consist of asphalt only :).
Trädgård is a garden attached to a house typically, like your lawn or back yard. Gård is more like a ’farm’.
Furthermore, a 'gård' can mean a 'country house', mainly stressing the house, with the rural setting implicit