Lack of practice is definitely the reason. I was writing my first CSS essay ever and it was in the examination hall....I encountered several problems with the outline, kept on changing the neat one till I finished writing the essay....I personally thought I went way off the topic for most part of the essay but Alhamdulillah passed the essay paper....grammar, expression, content, arguments do count.

My point is:these mistakes should be addressed before you walk into the examination hall.

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Whatever you do will be insignificant, but it is very important that you do it.
-Gandhi