1. Before singular and plural countable nouns when having mentioned it before:
Yesterday, a boy was found alive under the rubble in Turkey. The boy managed to survive
for 4 days without food and water.
2. When we refer to something specific:
The opinions of Dutch people vary widely on this matter./
The taste of this dish is a bit bland.
3. When we refer to something unique:
Yesterday, the king was visited by the US president.
4. When we refer to a period: the Middle Ages, the Renaissance.
5. Superlatives of adjectives: the best, the highest, the worst
6. With ordinal numbers: the first, the twentieth/ the 20th