No stackdump, I don't think so. I agree with xlbo that context is critical, and with tgreer that meaning and implication extends beyond the words themselves, especially with the spoken word where body langugage and intonation play a crucial role. But even with the written word, the differences exist.
==> Doesn't welcoming someone with "Hello" mean EXACTLY the same thing as "Hi"?
No, I think "Hello" is slightly more formal than "Hi". Put another way, I think "Hi" is a more familiar term. I don't answer the phone with "Hi" if I don't know who the caller is. In formal situations, I won't use "Hi" with the initial greeting. Similarly, when a good friend of mine comes into the office, I don't say "Hello". It's usually, "Hey", which to me, is even more informal, or familiar, then either "Hello" or "Hi".
==> Don't the two phrases "I came back yesterday" and "I returned yesterday"
Again, in context, they may not be the same. In golf, for example, the leader can "come back" to the field, but the leader doesn't "return" to the field.
==> In fact, isn't the list of words and phrases that can mean EXACTLY the same thing endless?, rather than being heavily confined as you suggested? I think this must be true, otherwise we would all speak the English language in exactly the same way.
I think the reverse is true. If all of these phrases mean exactly the same thing, why are there so many different phrases? I think people created new phrase because the existing one wasn't exactly right for the intended meaning. I submit there is very little confinement and it is from the subtleties that English earns its richness.
Good Luck
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