Work through one sentence step by step.
شَرِبَ الْوَلَدُ الْمَاءَ → The boy drank the water.
1. Put the past verb on فُعِلَ: dammah on فاء کلمہ and kasrah on عین کلمہ.
2. Omit the doer الْوَلَدُ.
3. Raise the former object الْمَاءَ to الْمَاءُ.
Result: شُرِبَ الْمَاءُ — The water was drunk.