what "under-club" means? and other phrase

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Yes, but Stableford or Stableford Bogey is much more complicated than i expected : O
what does it mean "competition against par"?
For example: "In this, the player takes seven-eights of his handicap against par, according to the stroke index. In other words, a 24 handicap would receive 21 strokes and gets one stroke on each hole plus a stroke on those in the index marked one, two, or three"
But how, if there's 18 holes? oh wait, he gets one stroke on each from 18 holes and because 21 minus 18 is 3, so 3 strokes are left, so we add them to the most difficult ones according to strokes index, so to three from the beginning to each? hahah, anyone understood?

"Counts two points for a hole completed in par or net par, one point for a score or net score of one over par...... etc" can we say that "net par", "net score" - par, which was adapted to the player?

You've got it.

He has 3 leftover strokes, so gets yet another stroke (so now 2 strokes) on the first 3 handicap holes. In other words, he shoots a gross 5, but nets a 3 on handicap holes 1, 2, and 3.

Nice!
erm, very short question:
"If the player is short, holds his hands low at address, or has clubs set up incorrectly, where the toe of the club is off the ground at address, the tendency is for the heel to drag into the ground and the clubface to close. As a result, the shot is pulled or drawn. In other words, the club is too upright."

How is it too upright? I mean, upright - more vertical than horizontal. if the club is held too low, the shaft is more horizontal... and so lie angle is smaller...

Please, could you answer as soon as possible?

Upright and Flat are lie angle terms. More upright = bigger lie number, flat = smaller lie number.

When the shaft butt (grip end) is moved lower, the angle between the sole (bottom) of the club and the ground is increased. Conversely, when the grip end is moved higher, the angle between the sole and the ground is decreased.