In a formal text, you should avoid using colloquial vocabulary, some of which would be used when talking to friends. This includes: gonna, ain’t, kid or to fire (dismiss).
However, if you are writing for a teen magazine or an informal website, using a few informal words can add a friendly effect to your writing. Note that a few words are the key. This is still a written text and overuse of informal words will annoy the examiner and make it seem like your text is more in a spoken style then a written one.
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Is your text technical or non-technical?
Technical vocabulary includes words which relate specifically to the subject about which you are writing. For example, music articles may mention the timbre of a voice, the lightness of a soprano solo in a performance or the allegro (fast) movement. Use technical vocabulary wisely and you will show that you are aware of your audience. For example, if you are writing for a newspaper about a topic you know the audience will be knowledgeable about, then technical vocabulary will connect you to them. If you are writing an advice piece that is meant to inform the reader about a topic, using technical vocabulary and explaining what it means shows that you know your subject but are also aware of the reader’s potential lack of understanding.