Written English allows us to employ many contractions that come from the spoken language. The following is useful advice about when to write using contractions:
“Technically speaking, contractions aren't necessary in written English. Using the full version of a word is always grammatically correct. However, there are a number of reasons why contractions do serve a valuable stylistic purpose. For example:
Contractions make your writing seem friendly and accessible. They give the appearance that you are actually "talking" to your reader.
When writing dialogue in a novel or play, contractions help reflect how a character actually speaks.
Contractions help to save space when preparing advertisements, slogans, and other written works that must be short and to the point.”
(from http://www.yourdictionary.com/dictionar ... tions.html
But even in informal writing, we must select when to, and when not to, contract; and my guiding principle is to think of how a sentence might have been spoken. Here are some examples to try out. For the most part, there are no right/wrong answers. It depends on the context you envision for the statement, and consequently the stress you believe the verb would receive in each utterance:
(1) You are not sure he was right, but I am.
(2) You are not sure he was right, but I am absolutely convinced he was.
(3) You are sure he is right, and I am sure he is wrong.
(4) You would think he would have gone straight home if he had not been feeling well!
(5) I changed my mind. I had thought that way until you proved he is right.
(6) There is no contradiction: I had agreed she would go first.
Try to give a short justification for each of your choices. Have fun!