..........a Lieutenant-General commands an corps (e.g. Engineer Corps) and a General commands an Army.
the engineer corps is not a good example of an army corps. while the u.s. army corps of engineers is famous for their military construction prowess, they are also well known in civilian construction. they are a distinct specialized arm of the u.s. army, much like the marine corps to the navy. a conventional army corps is made up of 3 or more infantry or mechanized or armored or tank divisions, or combos of any of 4 types of "fighting" divisions.
an army is not necessarily a composition of 3 or more army corps. it can be any unit size from brigade up depending on the country and its use. for example, papua new guinea may not be able to field 3 corps, but whatever pigmy or man-eating brigades they can muster will make up their army. only on the western and eastern fronts and the chinese and manchurian theaters in ww2 that we saw million-man formations reaching strengths of massive armies or army groups (3 or more armies) or even full tank armies.