Gerund vs. infinitive

Numerous exercises on gerund vs. infinitive along with their key are hereby made available for you to have access at your convenience.  You can also find the list of verbs used with gerund and/or infinitive here.

Here you can find more information and exercises on the same topic.

Mandatory, obligatory and compulsory

These words can indeed be used as synonyms in most contexts.  I also like the first response in the forum at the following link:
http://answers.yahoo.com/question/index?qid=20080507075259AAR2USG

Here is a link for another forum which can be useful, as well:
http://english.stackexchange.com/questions/2902/mandatory-obligatory-compulsory

How to Write Clearly

Here you can find the Commission guide entitled "How to Write Clearly".

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)

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) 

False friends for French

Here you can find a list of false between French and English.

Relative, -ed and -ing clauses

Here you can find some exercises on relative, -ed and -ing clauses along with the key.

Composite nouns

Some of the issues that come up in today's session concerning composite nouns can be found here along with some exercises.

Interinstitutional style guide and the Commission style guide

Here you can find the Interinstitutional style guide. 

"English Style Guide" released by the Commission can be accessed  here.

Latin terms, phrases, abbreviations and acronyms

You can find a list of Latin terms, phrases abbreviations and acronyms that can be used in English here.

PS: The abbreviation for the Latin phrase et alii is 'et al.' with a dot at the end.  Here is a general rule for the use of the dot in abbreviations:  If the last letter of the word being abbreviated is not the same as the last letter of the abbreviation, you need the dot.  What are the abbreviations for 'professor', 'doctor', 'mister' and 'abbreviation'?