Grammar

Since/For
http://englishlearner.com/intermediate/since-or-for-1.shtml

Prepositions

http://englishlearner.com/intermediate/prepositions-1.shtml

The definite article use
http://englishlearner.com/intermediate/the-definite-article-place-1.shtml

So or such?
http://englishlearner.com/intermediate/such-or-so-1.shtml