- "Key" and "important" both indicate notable elements.
- "Key" can also be used to indicate a required element, i.e. "keystone", however it is not always used in this way.
- "Key" can also be used to indicate "there is only one" though it is not always used in this way.
When "key" is meant as "singular & required & notable" it probably carries the most weight of all these words. However, since it is not always used in that way, I personally tend to give it less weight.
In casual use I would order them like this (strongest to weakest): crucial, vital, key, important.
In formal use I would order them like this (strongest to weakest): key, vital, crucial, important.
My favorite thesaurus, ChatGPT, agrees with my casual use ranking. https://chat.openai.com/share/10bb8195-c7cd-4c0c-935f-f5b4c3...
- Important: Describes that something has high priority, in a general sense. Very broad. Can replace any of the other terms, but is less precise.
- Key: Important in a utilitarian sense. Just like a literal key, using "key" here implies that something is an essential part of a solution to a problem/issue.
- Vital: Important in an ongoing sense. Think of "vitality"," the capacity to live, grow, or develop. We should use "vital" when we mean something is important to do as a habit, to maintain the strength of something.
- Crucial: Another term that is general. Basically "important" but with higher priority. It implies that there is some urgency, gravity, or necessity to whatever is crucial or the matter to which whatever is crucial.
I recommend searching the original text to see how they're used in context.
key - not used in the text
important - seems to be used when comparing things: more important or less important
vital and crucial - used in the text, both of them seeming to indicate the highest priority
But this strikes me more as a question of author’s emphasis rather than a ranking of the actual factors in reality?