I heard that we are not allowed to fast before Ramadan, is that true?

There are narrations that prohibit fasting in the second half of Sha'ban, except in two cases: if a person has a specific fasting habit, they may observe it, or if they started fasting in the first half of Sha'ban and continued it until Ramadan. Among the narrations are: "Do not precede Ramadan by fasting a day or two, unless a man is accustomed to a certain fast, then he may fast it," and "When Sha'ban is halfway through, do not fast." Fasting on the Day of Doubt is forbidden except for those whose customary fasting coincides with it. Fasting in the second half of Sha'ban, from the sixteenth to the twenty-eighth day, is disliked except for those whose customary fasting coincides with it, whereas fasting on the Day of Doubt or a day or two before it is forbidden, except for those whose customary fasting coincides with it.