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i din`t realise that i had made so much mistake on intonation during that exam, is there any way that i could improve my intonation?....or maybe it is cause by the unaccurate tunning before the exam.
Ignore the inaccurate tuning before the exam - bad workman always blame their tools
Your intonation will improve if you listen carefully to your own playing. I have many students who, week in week out still have bad intonation on tunes they know quite well. Why? Because they aren't really listening to what they are playing. The fingers just get put down on the fingerboard without a discerning ear.
To begin with, play a two octave scale of G major, play every note double-stopped with the open string next to the one you are playing on. Learn to hear the intervals, especially octaves - octaves with an open string are useful landmarks. If this seems difficult at first and you still feel you cannot 'hear' the notes, get someone to accompany you on a piano as you play each note.
Play slowly and listen, adjust each note as you go, don't do as many do, waffle through it convincing yourself it sounds fine and it must be everyone else who is tone deaf - I have students who would want me to believe that! In time your fingers will learn the distances between notes, and at the same time your ear will grow more discerning - therefore allowing you to adjust your fingers as you play and compensate for strings that may go slightly out of tune while you are playing!!