"Ago" is usually used in the simple past tense, indicating something happening in a POINT in time. For example, "the United States became a country 232 years ago". This sentence is in simple past tense, and the event "the United States BECOMING a country" happened at a specific point of the time line, which is "232 years ago". "Since" is used in perfect tenses, especially the present perfect tense, indicating something happening in a PERIOD of time, which started at some earlier time and still continues at present. The example "the United States has been a country since 1776", uses the present perfect tense to indicate the length of the period of time that the event "the United States BEING a country" has happened, and is still happening.

"USA has been a country since 1776" It's still a country. "Since" covers the whole period 1776-2008 "USA started 232 years ago" You're referring to that specific time (1776). "ago" doesn't include any later dates. "Bob has been gone since noon" Bob is still gone. "Bob left an hour ago" Bob may have left and came back.

Ago is much more general, and doesn't refer to a specific point in time. " A long time ago", or " Many hours ago". Since is more specific, saying that something has happened since a certain point in time. " It has not snowed in Chicago since last month." or you could say " It snowed in Chicago a long time ago.", but the second one doesn't reference what time that was.

I'd like to add that since is more like a lasting situation. (It started in the past and is still in effect.) You often see it somewhere in the emblem/logo of famous brands (e.g.): "Coca Cola, since 1886" Or they use it in a longer sentence: "Coca Cola, refreshing the world since 1886." Ago refers to a situation in the past. "The Romans used to live here 2000 years ago." "I called your father two hours ago." Good luck.

When you say ago, it is usually the last word in the sentence. Like you would say "I went to the store an hour ago" when you say since it is like "I saw three people since I got back from the store". Sorry if this does not help, I am a native speaker, I could help you if you wanted.