Comparing “guǒrán” and “jìngrán / dāngrán”
果然 vs 竟然 / 当然
How it works
These three words are very different.
果然 (guǒrán) means “as expected”. Reality matches prior expectation.
竟然 (jìngrán) means “unexpectedly”. Reality goes against prior expectation.
当然 (dāngrán) means “of course,” “certainly,” or “naturally.” It marks something the speaker sees as obvious or natural.
Example sentences
天气预报说明天有雨，今天晚上风一大起来，果然气温马上就降下来了。
The weather forecast said there would be rain tomorrow. As soon as the wind picked up tonight, the temperature indeed dropped right away.
谁也没想到，看上去最不起眼的那个人竟然在关键时刻发挥了决定性作用。
No one expected that the person who looked the most inconspicuous would unexpectedly play a decisive role at the crucial moment.
你既然已经认真考虑过了，别人提出不同意见，你当然也应该平静地听一听。
Since you have already thought it through seriously, of course you should also calmly listen when others raise different opinions.
Key point
果然 (guǒrán) = expectation confirmed 竟然 (jìngrán) = expectation contradicted 当然 (dāngrán) = obvious / natural / surely