I cannot, for the love of my life, understand the difference between
Don't they all mean the same, basically? Is there a difference, and if yes, how could I proceed to get it?
くだる both mean to move from a high place to a low(er) place. The difference is that the former focuses on the end point/result, whereas the latter focuses on the movement and/or the course taken.
さがる means to go down or back, often used in relation to some value or standard. You also use it when a part of something (bodily or other) lowers from its normal position.
Here are a couple of examples.
| おりる | くだる | さがる ―――――――――――――――――――――――――― 急いで山を～ | 〇 | 〇 | 小舟で川を～ | | 〇 | 舞台に幕が～ている | 〇 | | 〇 成績が五番～ | | | 〇 一歩後ろへ～ | | | 〇
Here's a screenshot if it's misaligned in your browser (courtesy of @psosuna)