The language you use in your essays is important - the use of 'academic' language and style adds authority to whatever you say. However, all of this effort can be undone if you don't also pay attention to the basic construction of what you write, in particular good grammar, use of correct spelling and punctuation and skillful selection of the right word for the job.
Spelling still matters, especially in exams. By all means let your word processing package draw attention to mistakes and misspelling in your weekly essays, but don't rely on automatic spell-checkers - it is well worth learning which words you commonly misspell and learning the correct versions. This is especially important when it comes to exams, where you will be hand writing your essays.
Choose the right words
More common than misspelling is the misuse of words (especially now, given the prevalence of word processing and spell-checking). One incorrect word can undermine the authority of your argument as your supervisor tries to work out what you really meant. For some, reading as widely and often as possible broadens your vocabulary and helps you remember correct spellings. If spelling is a particular challenge for you due to dyslexia, for example, then there are a range of techniques to help. If this applies to you then do refer to the guidance offered by the Disability Resource Centre.
Often a desire to appear sophisticated lies behind the choice of a word, term or phrase - but this can backfire in spectacular fashion if you get it wrong. Even if you have chosen your words well and polished your sentences to a high degree, you stand in danger of appearing to be more pretentious than sophisticated. It is much harder (and consequently more impressive when you manage it) to express complex ideas in clear and simple language. Don't be conned into thinking that unintelligibility is a guarantee of significance.
Syntax, grammar, punctuation
It is not within the scope of this resource to offer guidance on the English language and the sometimes exotic rules of its grammar and punctuation. There are numerous books on the topic such of which the following is just one:
Gordon Taylor, The Student's Writing Guide for the Arts and Social Sciences, (CUP)
It is important to be aware of the spelling, syntax, grammar and punctuation of your essay not out of a pedantic concern to 'get them right' (a hopeless task - language is constantly in a state of flux, and many 'rules' are disputed), but because the correct use of language allows you to give a sharpness and precision to your writing. The best of ideas will still have limited impact if clumsily expressed.