Common Mistakes in English are listed here.  Mistakes that have occurred often in this class are:

Accept/Except

Amount/Number

Anxious/Eager

Between/Among

Due to the Fact That

Economic/Economical 

Fewer/Less
 
It's/its

Like/as

Could of/would of

Than/Then

Well/Good

They're/their/there

Lexical Flexibility1 -   LexicalFlexibility2      There are more of these on this site, do more of them if you find you have problems with them.

Plurals    Mixed up sentences, very easy.    Verb tenses. 

There/their/they're v2 

Comparison adjectives  There are more of these on this site as well if you need them.

Much or Many?  Some or any?    Active or Passive?  Active to Passive.  Who Which Whose?  Self-pronouns. 


For more check these sources:  Better English.com  English Exercises Online  English Exercises