SURE, when used as an affirmation informally, can mean yes, okay, just so, naturally, of course, sure thing, etc. It is more common in AmE.
A: “Will you please go set the table before our dinner guests arrive?” B: “Sure, no problem.”
It’s also often used in response to requests for permission and to signal consent, or willingness to go along with something.
A: “Would you mind if I take the car?” B: “Sure, go ahead.”
However, sure only means ‘certainly’ when you say it with conviction. It doesn’t mean ‘yes’ by itself. When used flat, without any inflection, it’s neither yes nor no, it just says ‘Whatever’ i.e. ‘I’m totally uninterested’ or ‘I don’t know and I don’t care.’
“Do you want to go to the Art Fair in Amsterdam?” B: “Sure.”
Urban Dictionary defines it as “the quickest way to answer a question when not paying attention. Usually doesn’t include any thinking whatsoever”.
A: “I have to go to the store and then to the laundromat and then bla-bla-bla. ARE YOU LISTENING TO ME???” B: “Sure.”
It is often the response from someone who hemmed and hawed, but came down on the yes side eventually; what you may use when on the fence about something or you’re not too enthusiastic or pleased about.
A:”Honey, I think we should invite the Garcias.” B: “Erm…sure.”
You can also say sure sarcastically, to express the opposite: ‘I do not agree’, ‘I don’t believe you”, “Not really’, etc. Typically, these are used in informal circumstances when you want to be sassy or funny.
A: “Do I look good in these jeans?” B: “Sure.”
Although a truly sarcastic person is capable of making anything sound sarcastic, this one in particular rely heavily on tone and body language and is commonly used in response to nagging and stupid questions.
A: “Did you like my last post?” B: “Sure.” (not really)
Often, when said quickly, it stays true to the original meaning. When elongated, it is often an indiction of sarcasm.
A: “DUDE! you wanna crash the mall?” B: “Sure!” (non-sarcastic)/ “Suuure…” (sarcastic)
It can also be pronounced in a very high pitched manner, accompanied by a fake smile and sarcastic head nodding.
A: “Hey, do you want to come to the museum with me?” B: “SURE.”
Sure can also be used when someone is talking about a hypothetical situation; to acknowledge hypothetical ‘given’. We’re using our imagination here.
A: “So let’s say I gave you a thousand dollars. B: “Sure.” A: “And then you wouldn’t give it back.” B: “Sure, okay.” A: “Does that give me the right to kill you?”
As you can see, sure can mean much more than just certainty, and if you use it with the wrong tone of voice, you might be implying something you don’t necessarily mean.
SURE THING is also used very informally in the same way, typically in AmE.
A: “Will you be at the reception?” B: “Sure thing.”
If someone asks you for help, you can respond with sure thing to indicate certainty. It’s a done deal. You can count on it. No doubt about it. Consider it done.
“Yeah, I can help you move. Sure thing.”