Learn English – When to use “wherein” instead of “where”

Contrasting "a distinctive smell ... " to "a distinct smell....".

The former means that the smell is different from other smells. The latter means the smell is easily sensed. So, in a situation where there are many competing odors, a distinctive smell might not be distinct. For example, in a room where a lot of people are wearing perfume, each perfume is likely to be distinctive, but they would not be distinct.

When using "in case" as in "take this umbrella, in case it rains", you acknowledge the possibility of the action (i.e. rain). You're not certain it will happen, but it's not unlikely. You'll often see "in case" in instructions, rules, etc. - for example, "in case of fire, stop, drop, and roll".

When using "just in case", on the other hand, you emphasise the unlikeliness of the action. If you're "taking this umbrella, just in case it rains", you don't really expect it to rain, but you're taking precautions. It's also less formal, it's not something you'll see on a warning sign or in a rulebook.

"Lest" is somewhat poetic and archaic, but I think it puts a bit more emphasis on the undesirability of the consequences than the actual likelihood of the event. Don't quote me on that, though.