A/ an is called the indefinite article. It is used to mean any one of a general category. 

The is called the definite article. It means something specific. 

So:

A cat means any cat.

The cat means a specific cat that we know. 

A city could be any city in the world. 

The city means a specific city.

A is used when the noun begins with a consonant sound (not necessarily a consonant), and An is used when the noun begins with a vowel sound (not necessarily a vowel). Note that y can be a vowel or a consonant, and sometimes u has a y consonant sound. 

Between is used to compare two things.

Among is used to compare three or more. 

Ksama karen *aap

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I mean What is The Correct Use Of A, and The ? :) Is It True That English Language Basically Depend On Sounds ?  Also, What's The Difference of Usage Between ''Among n ''Between'' ? (Sorry if i'm Asking something too Elementry) :D