Often when I see a familiar shape, I feel I should complete it. I wonder is this a sign of good intuition or ability, or am I just slave to the form?

Do you have any recommendation for learning all of the 'proper shapes'? Obviously, there's no assurance that they're always correct, but it's still harder to know what to compare against if I haven't always seen the shapes.

If you are wondering, I suspect that it is the latter. But that does not mean that you should not play shape. For one thing, it is effective. Go Seigen once commented about how players who play honte, honte are hard to beat.

Step 4 Here, we can finally approach the level of professionals

Take the famous empty triangle that is good shape for example.

Perhaps it's a matter of semantics: a good move vs. a good shape.
For me, a good move is one that works, a bad move is one that fails --
this is independent of the artificial, subjective ideas of pretty and ugly.
If an empty triangle happens to be the best move, then by definition it is also both efficient and effective -- in that situation.
It might even be considered a brilliant and beautiful tesuji -- in certain situations.
Conversely, if a move fails, then by definition it is none of these: efficient, effective, good, or beautiful --
no matter how pretty-looking it may be, simply because it does not work.
I seem to remember an article in WeiqiTianDi in which something like this:

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was named dumpling of the century -- it occurred in a pro game, of course.
I wish I could find that issue.

But how do you know if a certain move will work or fail? Or even further, how do you know that a certain move has actually worked for you (even after the local exchanges are completed)?

I mean unless the position is simple enough to read all possible variations and unless the fight is a straightforward fight with a clear goal (like life&death), the definitions of "work" and "fail" are vague.

Therefore, in case you cannot read all possible variations, you need a guide to tell you which move is most likely to increase the probability of your future moves to collect more points for you then your opponent. I think only at that point the concepts of shape, efficient, effective, beautiful, etc come to play.

I don't think so.
Everyone makes mistakes, even pros -- only theirs are pro-level mistakes.
When we stop making mistakes of a certain level, we are ready to graduate from that level --
when we can finally avoid kyu level mistakes, we're approaching, well, amateur 1-dan, not pro. Not by a long shot.
A mindblogging gap separates Step 3 from 4.

Click Here To Show Diagram Code

[go]$$W
$$ X X
$$ X X[/go]

was named dumpling of the century -- it occurred in a pro game, of course.
I wish I could find that issue.

It seems to me that if you can finally avoid kyu level mistakes, you are approaching amateur 4 or 5 dan.

and less "authoritative definitives". I think false certainty is one of the biggest obstacles in SDK Go.

My understanding of good shape is that it puts you (your stones) in a better position than they were before. This can mean that they have better eyeshape, that they are thicker, or that they are just harder to attack. In some cases, these tasks can be performed rather simply by making a "good shape," such as the mouth shape. The relationship between stones in these typical "good shapes" is often advantageous. While playing the shape move may increase the probability of a local advantage, it is really more of a starting point - the first move to consider. But, as koosh points out, good shape is only good if it gets the job done, i.e., it's not enough for it to look good, it's effectiveness also has to be backed up by reading.

You mean statements like "An empty triangle is always an inefficient shape, wherever you play it"