When writing why shouldn't use the word very and a lot

Answer 1

Those words are overused and have become almost meaningless. Your writing will be much more interesting if you have more interesting words -- more specific and accurate words.

Here's a webpage that lists "plague words and phrases" and gives suggestions for avoiding them:
http://grammar.ccc.commnet.edu/grammar/plague.htm

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Answer 2

Those words are not specific. They have no real meaning. They are just something to throw in a sentence. How much is very... What is the difference between "I am very thirsty" and "I am thirsty enough to spit sand. " One is 'graphic' ; it gives you a specific image.

If someone gave you "a lot" of homework , how much is that? Are you going to have to work for an hour,or the whole weekend without stopping, or until your next birthday?

Communication needs to do just exactly that - convey a message - interestingly and specifically.