State vs. action verbs

Here is  a document where you can find information and exercises on state vs. action verbs, along with key for the  exercises. (Source: English Grammar in Use: Intermediate by Raymond Murphy)

Here is a link for a webpage where you can find some  exercises on the same topic.   

Here you can find the list of verbs we have looked at. (Source: LONGMAN Advanced Learners’ Grammar by Mark Foley &  Diane Hall)

Position of adverbs

Here are the main rules and exceptions on the position of adverbs in sentences:

First rule: if there is only one verb in the sentence, the adverb comes before the verb as in the following example: He usually comes to work on time.
Exception to the first rule: if the only verb in the sentence is 'to be', the adverb comes after the verb as in the following example:  He is usually on time.

Second rule: If there is more than one verb in the sentence, the adverb comes after the first verb as in the following example: He has already arrived at work.
First exception to the second rule: if the first verb in the sentence is an auxiliary verb with 'to', i.e., have to, need to, ought to and used to, the adverb comes before the first verb as in the following example: He usually has to start working by 9:30 AM.
Second exception to the second rule: if the first verb in the sentence is a contracted negation, the adverbs 'probably', 'possibly' and 'certainly' come before the first verb as in the following example: He probably won't come to work on time.

These being the main rules and exceptions, there is more to this topic.  You can find more information on this webpage, which also provides you with some exercises. 

Here is another document with exercises on the same topic.  You can find the key for the exercises here.

Tenses and key words

Here you can find many exercises on tenses.  I am also hereby making a list of all the tenses and key words available

Subject-Verb agreement

You can find a document on subject-verb agreement with exercises here.  The key for the exercises is available here.

Another document  is hereby available. 

Modal verbs

Here you can find some information and exercises on modal verbs along with the key for the exercises. (Source: English Grammar in Use. Intermediate by Raymond Murphy)
Here you can find more exercises on modal verbs.

Prepositions

Basic prepositions:  
Here you can find some exercises on basic prepositions.  The key to the exercises is hereby made available. (Source: English Grammar in Use: Intermediate by Raymond Murphy)

Noun, adjective and verb + preposition: Here you can find some exercises on nouns, adjectives and verbs which are used with certain prepositions.  The key for the exercises are also hereby made available. (Source: English Grammar in Use: Intermediate by Raymond Murphy). Here is a list of words with prepositions.

Phrasal verbs:  
Here you can find some exercises on phrasal verbs.  The key to the exercises is hereby made available. (Source: English Grammar in Use: Intermediate by Raymond Murphy)
You can hereby find a link for more online exercises on the topics above.

Four sources for audio and video streams

Here you can have access to the NPR of the NYC region.  It could be useful if you want to improve your listening comprehension of American English.  You can listen to the live FM broadcast or go their show archives.  I personally like Radiolab, Selected Shorts and Fresh Air among the shows they air.

Here you can have access to the BBC website, which also airs on the WNYC in the mornings Brussels time.  It could be useful to improve your listening comprehension of British English.  They also have links under "Learning English", where you can find numerous tools for learners of English as a second language.

Here you can have access to the audio and video streams on The Economist website.  This again is a good source for improving British English listening comprehension. I recommend using "The Week Ahead", which is released after 5:00PM every Friday.

Here you can have access to TED, which is a good source for numerous talks on a very wide range of topics.  Most talks are given by Americans and Canadians, but there are also other speakers of different nationalities, including some British.