(E2) I was then studying to be a doctor and (E3) I had met Sally already, whom (E4) I would go on to marry. (E1) I tried snails on May Day three years ago.

Perhaps looking at an extended example will help understand Reference Time (RT): CGEL, for instance, uses T r, вЂњtime referred toвЂќ, as approximately the same thing as ET, but its T o, вЂњtime of orientationвЂќ, and T d, вЂњdeictic timeвЂќ, are not distinguished the same way as RT and ST and it introduces an additional term, T sit, вЂњtime of situationвЂќ, to incorporate discussion of grammatical aspect. You should be aware that there are other models which are derived from ReichenbachвЂ™s and use similar terms, but in different ways. The advantage of this simple model is that it allows us to speak about time references in complicated utterances without confusing ourselves with terms like вЂpastвЂ™, вЂpresentвЂ™ and вЂfutureвЂ™ which we use to label both times and verbforms. There is more about this at our tag-wiki on tense. But the event you name, trying snails, lies in the past: ( ET < RT). Your reference time here, the time you are talking about, is the present: ( RT = ST). They speak of events which occur before the time you are pointing your camera at. вЂPerfectвЂ™ constructions, however, are relative tenses. You are pointing your camera into the past ( RT < ST) and speaking of an event, trying snails, which occurred at that time ( ET = RT). They speak of events which occur at reference time, the time you are pointing your camera at.

In the English tense system, the вЂsimpleвЂ™ and вЂprogressiveвЂ™ constructions are said to be deictic (вЂpointingвЂ™) tenses.

The position of the event you are photographing is the time at which the event occurs: ET.

The direction in which you are pointing your camera is the time you are talking about: RT.

(Note that in a conversation ST is ordinarily вЂnowвЂ™, but if you are reading an old text thatвЂ™s not the same as the вЂnowвЂ™ when you are reading it.

The position where your camera stands is the point in time where you speak or write your sentence: ST.

Think of your verb as a camera taking pictures of events on a timeline running from the past through the present into the future. These are what your text labels вЂSвЂ™, вЂRвЂ™ and вЂEвЂ™ I am going to call them вЂSTвЂ™, вЂRTвЂ™ and вЂETвЂ™ for consistency with other answers I have written on ELL. It involves three moments in time: speech time, reference time and event time. This is the вЂmodelвЂ™ for speaking about time and tense put forward by Hans Reichenbach in Elements of Symbolic Logic, 1947. That's pretty much all about ST, ET, and RT! (RT < ST)Д«) The event that he tried the snails ( Event Time) coincides with his Reference Time. His 'I tried snails three years ago' means that:ДЄ) His Reference Time is '3 years ago'. Thus, his Reference Time is '3 years ago'. In alternative B, 'I tried snails three years ago', the speaker recalls what happened three years ago by linking his mind to that time (the recalling/linking is indicated by the dashed arrow). (ET < ST)Д«) He uses Now as his reference point ( Reference Time) to look at the event. His 'I've tried snails' means that:ДЄ) The Event Time is before Now. He looks back (or thinks back) into the past, and recalls that at some point before now (indicating by the arrow), he was trying snails! The speaking time, also known as Speech Time, is Now. In alternative A, 'I've tried snails', the speaker speaks the utterance Now. In the picture, the Event 'I'm trying snails!' happened 3 years ago. The main point of the picture below (and of course, this answer) is to illustrate that we have more than one alternative to deliver our thoughts. (I apologize for the quality of my illustration.

This is probably best explained with a picture!

Www.edgestudio.com speech timer