Is a person considered a liar if he says, "I will not do such and such," and then does it, whether he was certain at the time of speaking or had forgotten what he said when he did it?

If you are informed of something you predominantly believe you will not do, and then it becomes apparent to you to do it, there is no harm in that, as long as you intended not to do it when you were informed. A Muslim is not considered to have broken a promise if they intended to fulfill it but were then prevented from doing so. However, if they intended to break the promise at the time of making it, that is reprehensible and the worst kind of breach. So, if you were truthful in what you intended to do at the time of speaking, your subsequent deviation from it does not harm you. If you are asked about your action, you must answer truthfully, unless there is a predominant harm in your disclosure, in which case you may use allusions (ma'areedh).