Halliday & Matthiessen (2014: 401):
Since the tense system is recursive, there should be no longest possible tense. However, in practice there are certain restrictions which limit the total set of those that occur. These restrictions, or ‘stop rules’, are as follows:
- (i) Apart from α, future occurs only once.
- (ii) Apart from α, present occurs only once, and always at the deepest level.
- (iii) Apart from α, the same tense does not occur twice consecutively. …
- following (i), we do not hear she is going to have been about to do it;
- following (ii), we do not hear he has been having done it;
- following (iii), we do not hear they will have had done it.
These restrictions limit the total number of finite tenses to 36.