"The sons eat in the restaurant."
Translation:Die Söhne essen im Restaurant.
1) "im" is already a contraction of "in dem" 2) "in the restaurant" takes the dative case (action happens at a location, vs motion towards a location)
It's not der sohne because it's plural, and plurals begin with die. Der Sohn is singular. It's not im das restaurant, because im is a shortened version of in dem. Keep in mind that the dative case (dem) follows after the word in. Another way to put it is that "im das restaurant" would translate to "in the the restaurant" plus is breaks a bunch of grammar rules.