Explanation of location 一般
In typical [ topic + opinion/ comment] structure:


「第一印象」(the first impression) is the topic
「一般(是)很難改變(的)」( generally, is hard to be changed) is the opinion/ comment.


If you write 「一般, 第一印象(是)很難改變(的), the adverb (一般) that should be followed by the verb (是) is now followed by a noun (第一印象) instead, and it is grammatically incorrect. 

*see comment from Enrico Brasil on your post:


  Adverbs usually are placed before verbs. 


The sentence has to be changed to 「一般(來說), 第一印象(是)很難改變(的)」

一般來說 = generally (adv) speaking (v)