Interactive Grammar Cube (tranformer mode)
In this mode Grammar Cube shows following details of English grammar:
1) Positions of tenses on a timeline (Past Perfect and Past Perfect Continuous -are the deepest tenses).
2) Dual meaning of Present Perfect. It is related to present (one side is connected to Present level) but it has a meaning of past (the other side is stretched to Past level). The same is with Present Perfect continuous.
3) Switching modal verbs (on top) show different groups of modals.