Another group of words that are probably best learned as a set are the ones that are related to things happening suddenly. And you'll notice they are similar in one way.
突然 (Túrán) is basically "suddenly" but can also be used for "abruptly" or "unexpectedly".
忽然 (Hūrán) is also "suddenly" or "all of a sudden". But the difference is that it's a bit softer. 突然 (Túrán) can be intended to be quite harsh or jarring. 忽然 (Hūrán) might be "sudden but not unexpected".
竟然 (Jìngrán) can also be "suddenly" but it tends to indicate more of a surprise that has happened, so more like "unexpectedly".
居然 (Jūrán) is also "suddenly" or "unexpectedly" and is closer in meaning to 竟然 (Jìngrán) but 居然 (Jūrán) is softer than 竟然 (Jìngrán).