If I would have realised (1)/ what a bad shape our library is in (2)/ I would have done something. (3)/ No error (4)

Answer

1

Explanation

Such sentences show that condition was not fulfilled in the past, hence job was not completed. The structure of such sentences is as follows: If + Past Perfect .... , ... would + have + Past Participle Hence, If I had realised should be used.