One concrete thing sits at the bottom: the foot (רֶגֶל). Watch what grows from it. A foot that walks the same path again and again wears a groove, and that groove is a habit (הֶרְגֵּל); the person who has worn it in is used to it (רָגִיל), which is also why רָגִיל means "usual". A foot that follows someone secretly is spying (לְרַגֵּל). The same three letters carry you from a body part to a routine to an act of surveillance, with no separate memorization. Every word here is verified against pealim, niqqud included.
The same root runs through the verb system. Each binyan is a voice: it bends the one idea into active, passive, causative or reciprocal. Tap to hear.
Notice the hif'il / hitpa'el pair: lehargil is causative (I accustom you), lehitragel is its reflexive (I accustom myself, i.e. I get used to it). The simple pa'al "to walk on foot" is archaic and not in everyday use; modern Hebrew expresses walking with other verbs, so the living verbs here are the habit-and-spying ones.
Same root letters, different noun moulds (mishkalim). The mould tells you the function: who does it, the act of doing it, or the thing produced.
Strip the prefixes, suffixes and vowels. Three consonants remain, and the meaning lives there.