Back e Ago: Qual é a diferença em sentido de tempo?

Qual é a diferença em usar "back and ago" em sentido de tempo?

These rituals may even extend decades back. “I Sent The Letter Two Months Ago.”
“I Sent The Letter Two Months Back.”

Por que "back" e não "ago"?

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Não há diferença em termos de significado, são sinônimos. "Back" é um pouco mais informal, apenas.
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Technically they are synonyms and can be interchangeable, though now always is the case. We can't say "long back" instead of "long ago", for instance (these are just grammatical minutes, ramblings of mine...)

When there is a difference, they establish them in the following way:
AGO - is about something that happened an amount of time before the present moment:
It's been ten years ago since I've seen you
It's been ten years ago since I had the chance to talk to you
It's been ten years ago I wonder what you're doing now...

From the Lyrics of song - Ten Years Ago.

The last time I went to English classes was six years ago.

BACK -
Would be about something that happened in a certain period in the past:
Back in the '80s, Freddie Mercury and Michael recorded “There Must Be More To Life Than This” but the duet wasn't actually released until 2014.

People loved to use jeans underneath skirts and dresses back in the '00s. Now they see their pictures (taken at the time) and they find them terrible!


Most of the time, the words BACK and AGO are interchangeable, though.
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“I Sent The Letter Two Months Ago.”
“I Sent The Letter Two Months Back.”
I would rather use the first sentence than the second (with back), more natural to me. You could even redo it into "I sent the letter two months now".

Just (be careful not to wedge the preposition "from" - "I sent the letter two months from now." (two month later/two months away). It would be impossible, at least for a while! Technology moves fast...

If you wish to maintain the "back", let's say you are sending an email now (month of June), then your choice of words could be as the following: "I sent the letter back in April" (two months ago, see?)