He is a good writer and he writes well – Adjectives and Adverbs

Some students struggle with the difference between adjectives (beautiful) and adverbs (beautifully), as this distinction is not always made in other languages. A good rule of thumb is the following: 
Adjectives usually refer to nouns, as in: “a nice lecturer”, “an interesting book” or “a busy afternoon.”
Adverbs refer to verbs and the way something is done: “she reads slowly”, “he answered badly”, “he listens carefully”, “she intervened successfully.”

For more detailed explanations and exercises, have a look at the following pages:

A very detailed explanation that is linked to two exercises.
http://owl.english.purdue.edu/handouts/esl/esladjadv.html

This page explains the use of adverbs and adjectives with the help of academic texts. 
http://www.utsc.utoronto.ca/~tlsweb/TWC/AdjsandAdvs.pdf

Quizzes to practise. 
http://www.learnenglishfeelgood.com/lefg3_adverbadjective.html
http://a4esl.org/q/h/lb/Adjadv.html
http://www.better-english.com/grammar/adjadv.htm
http://englischlehrer.de/tests/adjadv.php

Irregular Forms

Further problems arise from irregular forms and verbs that can be used both with adjectives and with adverbs.

Find a list of adjectives and adverbs that have the same form and adverbs that have two different forms. Linked to an exercise. 
http://www.eduhi.at/dl/adverbs_and_adjectives.htm

Another list of irregular forms with example sentences. 
http://college.hmco.com/english/raimes/digitalkeys/keyshtml/adjecti4.htm

This page explains why some verbs, such as ‘to look’ or ‘to smell’ can be used with adjectives as well.
http://www.ego4u.com/en/cram-up/grammar/adjectives-adverbs/adjective-or-adverb