Many statues and monuments have been eroded in . . . . . . . . last fifty years than had been in . . . . . . . . previous two hundred years.

An indefinite article is used when we are talking generally about something whereas a definite article is used when we are talking about something specific. The indefinite articles are 'a' and 'an'. 'An' is used before a word starting with a vowel or vowel sound. A is used before a word starting with a consonant or consonant sound. A definite article is an article that tells the uniqueness of something. 'The' is the only definite article. Both the blanks shall take 'the' as the sentence talks about a particular time period (last fifty years, previous two hundred years).