The definite article “the” is used to talk about something specific, something which has already been mentioned, and with superlative forms.
Where is the dog?
The computer in my office is broken.
I have a bag and a suitcase. The bag is very heavy.
The best teacher I have is Sue.
The indefinite articles“a/an” are used when we are talking aout something for the first time, or when we don’t specify exactly which thing we are referring to.
“A” is used before words which begin with a consonant sound, while “an” is followed by words beginning with vowel sounds.
I have a black cat.
I want to buy an umbrella.
No article is used when the meaning is general, with plural or abstract nouns, or with proper names.
Paolo is in Brazil.
Cricket is popular in India.
Food is expensive in London.
The use of articles can be different in other languages. For example, Italian students should be careful with the phrases:
I have a headache.
He has a temperature.