Use "the" when you are talking about something specific or something that both the speaker and listener know about. Here are some cases when we use "the":
1. Unique Objects: Use "the" for something that is one of a kind, like "the sun" or "the moon."
2. Previously Mentioned: If you've already talked about something, use "the" for that item later. For example, "I saw a dog. The dog was barking."
3. Superlatives: Use "the" with superlative adjectives, such as "the best," "the tallest," or "the fastest."
4. Common Places: Use "the" before names of some geographical names and with countries that include "kingdom," "states," or are plural.
For example: The USA, The Netherlands.
5. Instruments: Use "the" when referring to musical instruments, such as "the piano" or "the guitar."
6. Parts of the Day: Use "the" before parts of the day when they are specific, like "the morning," "the afternoon," or "the evening."
Remember, do not use "the" when talking about general things or when using uncountable nouns in a general sense, like "water" or "information."
Example: "Water is important for life."
