GRE deals with mainly on writing , quantitative, reasoning and verbal skills which needs to be given a high importance. For preparation my general tips would be practising alot on english grammer and mathematics. This would help to improve the persons analytical skills. My online suggestion would be to improve grammatical knowledge making use of Grammerly like websites, to distinguish between words on english you can make use of difference between sites like Listverse, to make familiar on mathematics would suggest Maths Test . Apart from these I would also would recommend to do a course on British Council which would help to improve your english language.

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