The biggest pain with understanding English Grammar
Where the structure with the 8 Formulas comes from.
Centuries ago, for unexplainable reasons, the term TENSES was introduced
into the set of rules describing the way English Grammar worked. Those rules have since been used for teaching/learning purposes.
Regrettably, the concept was absolutely wrong as with the introduction of Tenses, the NATURAL way English Grammar had been working and developing became invisible. Never since the moment the term Tenses appeared in the textbooks has it (the NATURAL way) been described in any of the coursebooks to have followed.
The main cause of the problem is that Tenses were not simply planted in the rules, they displaced the NATURALLY EXISTING term VOICE and started being used instead of it.
But what has dominated the English language grammar books for centuries is indeed alien both to the Grammar itself and its logic.
It is because of Tenses that the learners have to waste so much extra time (actually months and years) in a vain effort to understand how English Grammar REALLY works.