when we talk about something for the first time: I saw ... good film yesterday.
we speak about things in general: Water freezes at 0°C / Birds eat worms.
musical instruments: She plays ... piano.
meals: breakfast, lunch, dinner
when we speak about something that is part of a group or type: Do you want ... drink?
when the listener already knows which thing we are talking about because it was mentioned before: I'm going to take ... dog for a walk.
when we say what people's jobs are: He's ... architect.
towns, cities, states and countries: Russia, Moscow, Tula
when we speak about unique things: Here comes ... Sun.
sports or games: tennis, football, etc