Don't Use Abbreviations
Modern abbreviations, like "gr8" for "great" or "tbh" for "to be honest," are too informal for the IELTS writing test. Even abbreviations like "e.g." or "etc." should be avoided. These shortcuts skip grammar and vocabulary that you use to communicate in English and show that will prevent you from clearly showing the examiner what you are capable of as far as your use of the English language.
Not Too Informal
When we are sending messages to our friends, we often use shortened versions of a word like ‘pls’ instead of ‘please’ or ‘OK’ instead of ‘okay’.
However, you don’t want to do this for IELTS letter writing. Remember, it’s still a test of your English, so you need to show what you’re capable of.
In an informal letter, IELTS requires you to use full English grammar structures.
Also, you should use complete grammatical structures. Saying something like, “You going?” isn’t acceptable, and you need to use the full structure, “Are you going?”
Don't Use Idioms
Don’t use idioms in IELTS general letter writing as they will reduce your score. Idioms are quite old fashioned and are rarely used in modern English. Also, the circumstances in when they are appropriate are very rare, which means that even if you know what an idiom means, it will normally be inappropriate to use it.