Ex tunc and ex never are Latin expressions that mean, respectively, "from the start" and "since now", according to the literal translation into Portuguese.
These terms are quite common in the legal field, as they classify the type of effect that a given sentence or law will have.
When you say that a legal decision is ex tunc, means this will apply from the beginning of the process that gave rise to it, that is, of character retroactive, valid and affecting events prior to its creation, as long as they are directly related to the subject.
already a decision ex never is considered the opposite of ex tunc, as your application will start from the moment of its creation, not retroacting.
There is a lot of confusion about the meaning of both expressions among law students, especially.
To help not confuse the applications of ex tunc and ex never, there is a very easy “trick”: the ex tunc is associated with the "forehead", it's the ex never with the "never".
When someone is slapped on the forehead, the head tends to go backwards, a reference to the retroactive character of the
ex tunc. However, if you are slapped on the back of the head, the head moves forward, remembering that the ex never has “going forward” effects.See also the meaning of Hic et nunc.