What are the basic rules for using the articles

The indefinite article (a /an) is used with singular countable nouns referring to a non-unique item in general:

separate objects, people, ideas, etc.

singular

a microscope

a scientist

a plan

plural

five microscopes

a large number of scientists

several alternative plans

The definite article (the) is used with nouns referring to a unique specific item. A noun can have a definite article when

■ it is modified by a superlative or ordinal number_

the first experiment the last measurement the most significant results the only time

■ it refers to an entire type or species The telephone can be used to transfer data

■ it refers to an item previously mentioned_

They connected a phone line to a modem. The modem was connected to a computer in order for the computer to access the internet.

■ there is only one of something or it is fully specified by the context or background knowledge_

The periodic table is often used in chemistry.

The internet is now used by millions of people across the world.

The coefficient of expansion of brass is 0.000026oC.

The importance of international co-operation is emphasised in the report.

Note: Some generalisations may be needed in scientific use, in which case the is left out in long, complex, uncountable or plural noun phrases, in particular those including an of + noun phrase. In these sentences, both options, i.e. using the articles or omitting them, are correct.

(The) Little-known sources of air pollution are misfires in a car's engine.

(The) Creation of the simulation model allows for a degree of optimisation of (the) engine performance._

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