Dictionaries
Online dictionaries available through the University Library
The Cambridge Dictionary
The Oxford Dictionaries
Grammar
Your word processor may mark out certain grammatical errors such as disagreement between subject and verb, for example "they is" instead of "they are". Nevertheless, it is recommended that you also use a grammar book or website.
This page from the Online Writing Lab at Purdue University has recommendations on the most common grammar issues.
The following websites provide grammar exercises:
Cambridge Assessment
British Council
The Academic Phrasebank
The Academic Phrasebank is a collection of phrases that can be very useful in academic writing.
Synonyms
Synonyms may be used to create variation in your writing. However, you should not attempt to find synonyms for subject-specific terms. Also be aware that there may be subtle differences between similar words, such as "write" and "record".
This online resource can be used to find synonyms.