Islamic law does not sort acts into permitted and forbidden. It sorts them into five, and the scale is what gives the system its texture. The obligatory is rewarded when done and blameworthy when left. The recommended is rewarded when done and not blamed when left. The permitted carries neither. The discouraged is blameworthy when done but not punished. The forbidden is blameworthy when done and required to be left, and refraining from it is itself the obligation.
The scale means that a great deal of religious life falls in the middle three categories, where a Muslim is being advised rather than commanded, and that describing a practice as haram is a strong legal claim rather than an expression of disapproval. Jurists are correspondingly careful about it, and a recurring criticism within the tradition is of people who reach for the strongest category when a weaker one is what the evidence supports.
There is a further refinement that catches readers out. The Hanafi school splits the forbidden in two: haram proper, established by evidence that admits no doubt, and makruh tahrimi, prohibitively discouraged, established by evidence that is strong but not decisive. The consequences differ, and a ruling that a Hanafi jurist places in the second category will often be reported outside the school simply as forbidden. A separate question is whether a forbidden act, once done, is legally effective, since a contract may be sinful and still valid, or sinful and void, depending on the case.