In formal language, we would write cannot, and with other such forms, we would use two words- must not, should not, etc. In informal writing it's fine to use can't. On most forums, people don't write formally; they tend to write something that is between speaking and formal writing- it follows the rules of grammar, but is relaxed about things like contractions, but not about the sort of abbreviations found in text messages because they can be difficult to read in texts longer than a few words.
PS If you want to ask a separate question, please start a new thread because it can get very complicated when there are a number of different questions in the same thread. ;-)