There are six.
Impossible: Literally. Not a slang term for "very hard". Some people will say that nothing is impossible. This is obviously not true.
Somebody: Nothing is impossible!
Me: Can you walk to the moon?
Somebody: Of course not!
Hard: Possible with effort, time, concentration, creativity, energy. Example: Flying to the moon.
Tedious: Requiring time and energy, but not creativity. Example: Flying to Florida.
Easy: Nearly effortless. Apparently requiring little time or energy, yet accomplishing a lot. Example: Calling Florida on the phone.
Automatic: Effortless. Takes care of itself without attention. Example: Answering machine.
Invisible: Out of one's hands. Forgotten. Examples: Eyesight, digestion, electricity, democracy.
Individual skill development, and technological progress, means pushing tasks down this scale. Advances in knowledge make the impossible merely hard. With further development and practice, hard tasks become tedious. Tedious work is speeded up and becomes easy. Easy work becomes easier and easier until it's no longer work, it's automatic. Automatic tasks are forgotten and become invisible, as we focus our energy on the hard and tedious work we still have.
This leads to some aphorisms:
From a workflow standpoint, inefficiency comes from not knowing the degree of difficulty of a task. If you overrate the difficulty of a task, you're doing it the "hard way". The worst case is to mistake a hard job for impossible, and not try at all. On the other hand, if you underrate a task's difficulty, it probably won't end up done right. For example, if you think something tedious is easy, you won't dedicate the required time to it.
Getting from impossible to hard often depends on external developments. The impossible needs to revisited occasionally, to see if other developments have moved the task to hard. You can't make vaccines until you know about the immune system.
Working on the impossible without expecting it to be accomplished grows knowledge that might be useful later.
Don't mistake your own fear of difficulty for impossibility. Don't mistake your own impatience with tedium for difficulty. Visualize non-impossible stuff getting easier.
It is hard to determine if a task is hard or impossible.
Automatic and invisible tasks need to be periodically looked in on.
That is all.