Explanation of location 一般 My question is regarding the following sentence.

第一印象一般很难改变。

I can currently not figure out why based on the grammar 一般 is placed at the position it is in the sentence above. 

Could anybody shortly explain with grammar why it is located there and for example not at the beginning of the sentence (sounds logical to me).
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)