Without reading the title of this post, if you were asked to give a synonym of “sure”, wouldn’t your answer most likely be “certain”? And vice versa? In most cases, these words are identical and interchangeable. But these cases are mainly cases in which we are describing a person (I, you, he, she, etc.) with one of these words. “I’m certain/I’m sure,” “Are you certain?/Are you sure?” However, I hadn’t realized until recently that these words aren’t, in fact, perfect synonyms in every possible instance, which means they aren’t perfect synonyms at all. If you want to say whether a situation or contingency is certain, you can’t say “sure”: “The Devil Rays and Yankees are still fighting for the AL East title, but it is certain that they’ll both be in the playoffs.” “I know which apartment complex I’m moving into, but the exact unit is not yet certain.” If you used the word “sure”, at least in the English language that I know and grew up speaking, it would not sound right and people might wonder, “What? Who’s sure? It sure is what?”
You might say, “Oh, the explanation is simple: ‘sure’ describes a person’s state of mind/attitude, whereas ‘certain’ can describe both people and non-personal nouns.” Not so. With the same pronoun “it”, we use the idiomatic expression “it’s a sure thing” but would never say “it’s a certain thing”. Just another reason why English, like all languages, is both beautiful and weird.