Classification versus identification meaning of articles

Classification

Use an indefinite article (a/an or Ø) to classify a noun and show that it represents a type, group,
or a class distinct from some other type, group, or class.

  • An earthquake (a type of natural disaster) struck at 7:11 AM.
  • A gas (a type of gas) that can be deadly is carbon monoxide.

Do not use an article for plural or uncountable nouns:

  • Ø Stars (celestial bodies) shine brightly.
  • We expect Ø complications (additional problematic conditions) while they are sick.
  • Have you ever seen Ø traffic (passage of vehicles) like this?

Identification 

The definite article (the) can identify a noun and show that it has been singled out in some way. 

Generally, the speaker or writer knows the listener or reader is aware of the noun because it was
previously mentioned or they can see it, has heard of it, has experienced it, has read about it, etc.

  • The movie (you heard about it) stars Will Ferrell.
  • The earthquake (you know about it) destroyed many buildings.
  • The gas (you smell it) can be harmful.
  • The medical complications (you experienced them) were unexpected.

  • The traffic (we are riding in it) is dangerous.

Special uses of the definite article (the)
Use the:

with unique nouns

The sun is very bright.

The universe has more stars than all the grains of sand on the world’s beaches combined.

before superlatives

The most significant effect occurred in June.

 before ordinals (first, second, third, etc)

The third component was missing.

 before modifiers that make the noun that follows  specific (same, sole, chief, only, single, solitary, main, etc.)

They are referring to the same theory.

The digital audio recording system at SFU is enabling technology to take university lectures out of classrooms and put them into the pockets of students.

 in phrases that refer to a specific part of a whole group

Each of the experiments was successful.

Half of the population suffered greatly.

with identifiable nouns that are followed by a modifying of-phrase

The effect of an earthquake can be felt for miles.

The total budget of the average national AIDS program in the developing world today is less than the medical cost of caring for only fifteen people with AIDS in the United States.

before adjectives that represent groups of people

A major problem is caring for the homeless (people).

The Red Cross is committed to helping the needy (people).

with certain nouns, such as mechanical inventions and devices, to refer to a general example of something rather than a specific object the speaker/writer has in mind

I always listen to the news on the radio when driving to work.

They took the bus to work today.

before locations associated with certain typical or habitual activities. The listener/reader may have no idea of the exact location to which the speaker/writer is referring.

I’m going to the gym after class.

Have you been to the beach this summer?

They need to pick up some groceries at the store.