vi. If I had not watched him, he would have gone without food himself in order to feed the refugees.
what is the meaning of it?
He wanted to give his food to the refugees. However, I watched him and would not let him. If I hadn't been watching him, he wouldn't have eaten and the refugees would have had his food. The use of 'himself' shows that the refugees went without food.
vii. The best thing to do is to have an honest study of words and memorize those given in the previous examinations.
change to studying and memorizing?
Not unless you change other parts of the sentence too. Simply substituting the 'ing' form is ungrammatical.
viii. So far in our teaching of languages, we have given enought emphasis to some common errors, but we have not stressed the complicated problems.
shouldn't it be emphasis on? No, emphasis to is fine. We have had an emphasis on, but we give an emphasis to.
ix. At that time, all I knew that he was wrong.
that=> was? ...was that he was wrong
x. As the examination is drawig near, all the students, in my class are busy preparing their lessons.
can I use 'busy to prepare'? No. No comma after students either.
xi. Once, malaria was a deadly disease. This is especially true of some developing countries.
of => in? Either is ok.