Don' do that!

  I did not know that there are many grammatical rules in writing formal English. To make a paper reader-friendly, writers have to follow several codes.  Most of them were maybe told by Rab in the last class and I was surprise that I have violated many of them, but what made me shocked most was the rule that successive sentences should not start with the same word, because I had broken it many times. However, solution is very easy: just turning them into one sentence is enough. This also helps readers to catch flow of a passage and probably this is the reason the rule exists.
 Though this is not much concerned with this topic, I have not yet understood the difference in use of "a" and "the".