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Need help differentiating two similar f2l cases

Discomantis

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So I recently learned advanced f2l and I am having trouble with the case where the white sticker on the corner is facing up and the edge is on top and they are split. I don't know a fast way to pair them up without moving the edge to its corresponding center to figure out which algorithm to use. How do I know which of the two algorithms to use by immediately looking at the pair? (and how you can solve the pair into the back slot if possible)
 
Can you send a picture through here for a clearer explanation on which cases you're talking about? There are quite a few cases that fit your description...
 
He's referring to cases 19, 20, 21, 22 , which all look rather alike.
Gotta admit, I struggled with these. The easy cop-out is indeed to AUF until the edge matches the centre, and it's not that harmful, movecount wise. I eventually got good enough at recognition, and no longer considered these cases a problem. It was when I tried to learn pair-making using empty slots that the struggle reappeared. And I never did get it mastered ... too long recognition, too many errors.

As an example:
Starting with a completed first two layers, setup:
F' U' F R' U2 R
The most obvious thing is to recognise that a U will align the edge and centre, then solve it algorithmically, so typically
y' U R' U2 R U' R' U R; = 9Turns in QTM

But that's not really recognising the case, if I could do that consistently, I would see the empty-slot solution:
R U2 R y' R' U R; = 7Turns QTM = better

In this specific case, I can see the solution... but if the setup was F' U' F R' U2 R U' , I would bale out, and start with a U2, giving me a 10T solution, not great.
 
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If you learn to recognise EO, it gets a little easier
If the U facing colour of the edge is the same as the F face, it is oriented and you won't need to rotate to solve it
If not, the edge is misoriented and the pair can then be solved after a y or y' rotation
 
It's largely down to practice, really. For each of these 4 'F2L' cases, there's 16 variations ( 4 y positions, 4 AUF possiblilties ). Just go through them all, and gradually you'll recognise more and more of the cases.
For example, take the cases where the AUF is such that the edge is correctly aligned with the centre.
Case 21 setup: R U R' U' R U' R'
if you add y' , so it's one of your 'slot at rear' situations, you could simply do a y and reverse the setup, or do y' and execute the reverse of the setup on L & U ....
but there's a slightly neater way:
R' U' R y R U R' or R U' R y' L U L'

Likewise if the setup was followed by y , so the edge is at the front;
Solutions could be:
L' U' L y' R U R' or L' U' L y L U L
The last one is part of my OH repertoire, executed as z U R U' x' U' R' U ( z' )

Also from the original setup, without y moves, the pair can be made with:

L U' L' then R U R' to insert, if the LR slot is empty.
 
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