REALLY struggling to grasp op amp theory!

[BrianHG]

[Jan Audio]

Also, this video is extremely good:

Yes i am learning from this video, only i have 1 question, the maker says : this are my rules.

There is no current flow into the opamp inputs, only what happens if you take a NE5532 ?, it is not a FET opamp like the TL072.

[TimFox]

With a bipolar input such as the 5532, there is a small but relatively constant current flowing into the two input terminals.  This has an effect on the DC (quiescent) voltages in the circuit, but almost no effect on the AC amplification.  The 5532 was designed specifically for audio applications, where DC stability is less important than distortion and noise.

[rstofer]

Input bias current does exist but may be quite low.  Your N5532 has horrible offset current so it must exist for some reason other than precision circuits, likely audio.  The venerable LM741 has similar input bias current and it's been around since '63.

In any event, there are techniques for minimizing the effect:

http://blog.parts.io/whats-all-this-input-bias-current-anyhow/

http://www.ittc.ku.edu/~jstiles/412/handouts/2.7%20DC%20Imperfections/Reducing%20the%20Effect%20of%20Input%20Bias%20Current%20lecture.pdf

The simplified rules get you in the ballpark.  They are "slide rule" calculations, not LTspice models.