Could someone explain to me why with kanji like 生 after the top horizontal stroke I have to go on with the vertical one, while instead with kanji like 金 I have to draw the two horizontal strokes and then the vertical one? Is there a simple rule to be applied?
The primary reason the stroke orders are what they are is that in Japanese, lots of characters are optimised for writing efficiency and/or cursive script. While they seem illogical at first, if you follow these stroke orders, you will eventually start writing more efficiently with less tired hands for longer pieces of writing (while simultaneously maintaining legibility).
This is「生」written in one or two strokes, which can only be achieved if you follow the hand motions of the Japanese stroke order.
This is「金」written in three or four strokes. Again, it follows the hand motions of the stroke order that you're taught.
「全」is no exception!
The secondary reason is that if you don't follow these stroke orders, you risk getting marked wrong in your tests.
All images from cidianwang.com and jisho.org.
This site here may help: https://www.tofugu.com/japanese/kanji-stroke-order/
Summary: Kanji Stroke orders are:
- start left-to-right, top-to-bottom;
- then horizontal lines coming before vertical lines;
- then symmetrical lines on either side of a vertical line;
- then symmetrical strokes not on a line;
- then inside-to-outside (unless it's a box kanji, then box it in before writing the rest);
- then right-to-left diagonals;
- then line that cross a lot of other lines;
- then dashes at the end unless the dash is the top-left line, in which case it comes first.
Knowing these stroke orders can help you write faster for longer. The only other place I can think this is useful in is school if they make you write them step-by-step.