years ago

If you say "пять лет тому назад" What is the significance of тому? What's the difference between saying this and just "пять лет назад"?

Can't think of anything rather than this:

Пять лет тому назад. Тому might refer to some event that itself could happen at any time stamp. So this phrase can be heavily used in a story telling to tell about an event that happened within the story's time frame.
Пять лет назад. Well it's just five years ago.

All in all these two are pretty much interchangable, both can mean the same thing.

Think of the dative Тому like in the phrasing мне ____ лет. It's saying to "that" five years.

I had to think about this one, but after looking up назад on Russian Wiktionary (Vikislovar), here's my explanation for why the dative тому is often used with it. When назад is used with the meanings "towards the rear" or "back to the original source," it is considered a "наречие" -- i.e., an adverb. But when it's used in the meaning "ago," назад is regarded as a "предлог" -- a preposition, logically meaning "behind" or "in back of".

And prepositions, by their very nature, need a noun or pronoun to come after them (although English is much less strict about this rule than Russian or Latin!). So тому is often put in there for the "prepositional" назад to go with. In other words, "птяь лет тому назад" logically means "five years in back of THIS present moment."

P.S. I checked an English dictionary, and "ago" is definitely considered to be an adverb (or, in some situations, a predicate adjective), but not a preposition.

By the way, here's an image showing назад in its adverbial use, with the sense "returning towards the point of origin":

https://upload.wikimedia.org/wikiped...ee-blu-ray.jpg