Introduction
A, an and the are called articles. They are really a kind of adjective: they come before a noun (or before the adjective of a noun) and tell us whether we mean any one thing of its kind or one particular thing. There are only three of them, but deciding which one a gap needs — or whether it needs one at all — depends on the noun, the sound of the next word and the meaning of the whole sentence.
The two kinds
A or An: go by sound, not by spelling
Rule 1Use 'a' before a word that begins with a consonant sound, even if its first letter is a vowel.
a + consonant sound
Rule 2Use 'an' before a word that begins with a vowel sound, even if its first letter is a consonant.
an + vowel sound
NoteThe letters F, H, L, M, N, R, S and X are pronounced with a vowel sound at the start (ef, aitch, el, em, en, ar, es, eks), so an abbreviation read letter by letter from them takes 'an'. But a BA, a BSc, a UN officer (bee, yu).
NoteThe article goes by the word that comes right after it, which is often an adjective, not the noun: a man → an old man; an apple → a big apple; an hour → a full hour.
How the question is set
In the SSC exam a short passage is given with gaps, usually ten, marked (a) to (j). You write the article each gap needs, and a cross (×) where no article is needed. A cross is a real answer, and a passage almost always has one or two of them.
How to solve a gap
Work in four steps. (1) Read the whole passage first, so you know what has already been mentioned. (2) Look at the noun after the gap: is it countable or uncountable, singular or plural, a proper name or a common noun? (3) Ask whether it means any one of its kind or a particular one — mentioned before, made particular by a phrase after it ('of …', 'who …'), or the only one of its kind. (4) If the answer is 'a' or 'an', listen to the sound of the word right after the gap and choose.
NoteA singular countable noun can never stand alone. 'He is doctor' and 'I saw dog' are wrong; it needs a, an, the or a word like my, this or every.