These are my opnions, and they might not be 100% since I'm not a native speaker. Obviously = when you're pointing out information that is already known or should be known to the person who's listening (in your subjective opinion) // "[you said the cat wouldn't be here when we got back, but] you're obviously wrong [since the cat is still here]." Definitely = when you're sure that what you're talking about has no mistakes (your opinion is that the correctness of what you're talking about is objective and absolute) // "The cat is definitely still here." Absolutely = when you want to show very strong agreement, disagreement to (or conviction in) something; usually used in a situation where you agree with someone else; relatively rare usage; has a similar but more 'objective' feel than 'obviously'. // "what you said is absolutely wrong." Exactly = used in two ways. 1) when something happens at the same time something else happens, no more and no less. 2) as a slightly weaker version of 'absolutely'. // "I made it to the bus exactly when it was leaving." | "that's exactly it, you are absolutely wrong."

I can't explain them but I'll try to explain it through examples: exactly: (in two weeks it's my birthday and I wanted to get book A), two weeks later, you get your birthday present and guess what it's book A. So you say: Wow, it's exactly the book I wanted, thanks mom and dad. absolutely: You are eating a really good green tea cake so you say: I absolutely love this cake! definitely: Okay, so you have a due date for a paper you have to write. The due date is November 1st. The teacher would say: You definitely have to hand in the paper by November 1st. obviously: your friend was riding her car when she got into an accident. You asked her what happened. She says: I dont know, I was just texting my boyfriend and all of a sudden a dog crossed the road,.... You say: Well, obviously, you know you can't text and drive.

I believe it would depend on the context of the sentence as the words can have different meanings in different sentences e.g. Person 1: "I am absolutely sure I bought this for exactly £10" Person 2: "But... Are you definitely sure?" Person 1: "Obviously! It said it on the price tag!"