A correspondent writes to ask whether it is possible to use sentences like I've been studying English since four years ago and Since three years ago, I've had several accidents.
It must be possible. The question wouldn't be coming up at all if people weren't being heard to use such sentences. What has happened, of course, is that there is a clash between the usage and the rule that is widely taught in grammar books.
The rule says that since and ago are incompatible, because since refers to an event that has current relevance whereas ago refers to a completed event in the past. Another way of putting it is when teachers say that ago is looking from the present towards the past, whereas since is looking from the past towards the present. There appears to be a clash of logic: people can't be doing the two things in the same sentence, goes the argument.
But of course they can. It's the same issue that arose when I discussed ago with present perfect have in an earlier post. It's perfectly possible to switch or conflate different points of view in a single sentence, especially in speech. And people have been doing this with since/ago for ages, both in phrases and clauses. In As You Like It (2.7.24) we hear Jacques reporting Touchstone saying 'Tis but an hour ago since it was nine'. In 1633, a character in John Ford's play The Broken Heart (3.5.63) says 'Tis long agone since first I broke my heart'. There's also a parallelism between ago and since which dates from the Middle Ages. Here's an example recorded in the OED, from Caxton (1489): 'Long time since... shee fell sick and died'.
The usage isn't as illogical as traditional grammars suggest. The logic goes something like this:
I haven't seen John since 2009.
2009 is a year ago.
Therefore, I haven't seen John since a year ago.
And this is how we most commonly hear the usage.
Every day, since a year ago today, I've been writing in my diary...
I haven't been this happy since a year ago.
Retail sales figures show consumer spending trend at highest since a year ago.
Costs have halved since a year ago.
There are over 2 million hits for 'since a year ago' on Google, several in quite formal written contexts, such as financial reporting. So, I have to conclude that there are two rules in English relating to ago and since, not one:
I want to say that a completed event in the past, expressed through ago, has no current relevance: I don't use since.
I want to say that a completed event in the past, expressed through ago, does still has current relevance: I do use since