The meaning of ‘good english‘, and hence writing good english, may vary from people to people around the world since we come from different backgrounds and due to the nature of our education. Despite the fact that there is no governing bodies of linguists to gauge its exact meaning, we do have a standard set by the authorities indulged in linguistics. Hence, sensible authors, careful writers and educated people using english language as their medium of writing, try to conform to these standards as closely as they can. These standards may be grammatical rules, language structure, and universally accepted words, idioms or metaphors.

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Yes.

If you do not express yourself accurately and correctly, then you will almost certainly be marked down.
To continue to have particular difficulties, then seek further help and advice, does provide a support service for those who have specific problems.

Spelling - very visual. The ways it sounds is not always the way it is spell.
Punctuation - organize a text and make it easier for a reader to follow.
Register and Style - precise and economical in the use of words.
Discourse Markers - promote the coherence.
Sentence and Paragraph Construction

Lastly, Review your works. If you have the outlined. Then start writing that focus on your features.