أَكَلَ الْوَالِدُ وَالْأَبْنَاءَ
The father ate with the sons.
- الْوَالِدُ is the doer of أَكَلَ.
- The waw gives the meaning with.
- الْأَبْنَاءَ identifies those who accompanied the father in eating.
- Therefore الْأَبْنَاءَ is mansub as maf'ul ma'ahu.
A plural noun can also fill this role; maf'ul ma'ahu is not limited to a singular companion.