Teacher: Today we’re going to talk about tenses. Now, if I say I am beautiful, which tense is it?
Student: Obviously, the past tense.

English tenses are a continual source of bewilderment for students and teachers, so here is a handy table (a=active, p=passive).

The verb shag is regular—but don’t forget to double the g!

Now try the tenses matching exercise

The major problem is quite simply one of grammar, and the main work to consult in this matter is Dr Dan Streetmentioner’s Time Traveller’s Handbook of 1001 Tense Formations. It will tell you for instance how to describe something that was about to happen to you in the past before you avoided it by time-jumping forward two days in order to avoid it. The event will be described differently according to whether you are talking about it from the standpoint of your own natural time, from a time in the further future, or a time in the further past and is further complicated by the possibility of conducting conversations whilst you are actually travelling from one time to another with the intention of becoming your own mother or father.
Most readers get as far as the Future Semi-Conditionally Modified Subinverted Plagal Past Subjunctive Intentional before giving up: and in fact in later editions of the book all the pages beyond this point have been left blank to save on printing costs.
Douglas Adams, The Restaurant at the End of the Universe