A
before a countable noun beginning with a consonant ( any letters other than a,e,i,o,u)
An
before a countable noun beginning with a vowel( a, e, i, o, u)
The
before a thing or a group of things which is the only one or only group that exists
before the names of seas, oceans, rivers, deserts, mountain ranges, buildings, hotels, theaters, restaurants, special means of transport, countries
before a superlative ( the highest degree of comparison of an adjective)
e.g. hottest, thinnest
before a particular noun or a noun that has already been mentioned
countries' names, the is used only when they are formed of more than one words or when they end in s
e.g. The United Kingdom, The Philippines
For names of cities, towns, districts, roads, streets, "the" is not used