Roughly, it's the logarithm (base 2) of the number of guesses that an optimal password guesser would have to guess in order to guess your password. It's a measure of how unknown your password is.

> it has so many possible combinations, that every speaker is continually creating sentences that have never before been uttered.

Yes, this is why all of the suggested alternatives like "choose a line from a popular song" are so much less secure.

You do lose a little entropy that way: you've merged "great" and "greatly", and "correct" and "correctly", suggesting that your modification process considers adjectives and their corresponding adverbs as equivalent. If those examples are typical, that removes one bit of entropy. But you've probably added more than that back in, if your choices of "a", "will", "the", and especially "inserted" are unpredictable. (Alternatives might include "the", "would/did/could", "a", and "removed".)