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I have a sudoku puzzle that I have been doing for 270 hours. I never guess numbers and always thought that every move in sudoku had to have a logical explanation. Until the app generated this random puzzle:

enter image description here

I've never used any kind of help from an internet, but now it seems the time has come. I watched some videos, looked into some guides (i.e. sudokusnake.com). And they only helped me to eliminate 9 as a candidate in I5, G8 using the empty rectangle technique. The current state of my sudoku is:

enter image description here

What can be done to solve this sudoku puzzle, other than guessing? Any tips/hints?

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    $\begingroup$ Are you 100% sure that the numbers you've written are correct? $\endgroup$ – Eagle Mar 10 '19 at 13:15
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    $\begingroup$ I am starting to solve this, so just wanted to confirm to decide if I should start from the beginning, or from the state of the second picture. $\endgroup$ – Eagle Mar 10 '19 at 13:49
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    $\begingroup$ Maybe some of these strategies might help to eliminate some candidates. $\endgroup$ – Kevin Cruijssen Mar 10 '19 at 14:33
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    $\begingroup$ I have written my own Sudoku solver. It implements level 1 techniques (single candidates and other forced moves, direct eliminations etc), and level 2 techniques (naked pairs, hidden pairs, x-wing etc) but not level 3 techniques (naked/hidden triples, swordfish, etc). The level is essentially how many moves it would take to reach a contradiction if you were to make a wrong guess instead of the move the technique tells you. To solve this sudoku my solver needed to guess at the point you got to, so your next step must be at least a level 3 technique. $\endgroup$ – Jaap Scherphuis Mar 10 '19 at 16:02
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    $\begingroup$ This solver shows a step by step solution. It uses some hard techniques, but finds a solution without guessing. $\endgroup$ – Kruga Mar 11 '19 at 10:01
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We can say F1, F3 is 9,8 because it is fixed regardless of whether we pick 3 or 4 for A4.

because:

If we choose A4=4, then A3=9 and so F3=8.
For A4=3 the case is (a lot) more extended, but only uses simple eliminations.
First D4=9, E5=3, E3=7, E8=9.
Then D2=3 and so G2=2 and I2=1.
From E5 and I2, I5=6.
From E8 and I5, I8=4.
From I5 and I8, I1=8 so F1=9.

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Hint (a little bit of guessing :-)

If you put 4 into A3 and go through simple elimination steps based on the options you have already figured out, you'll see that it was a wrong move. So A3 must be 9.

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