This root carries two senses that turn out to be one. The pa'al זָהַר means to shine, to glow, and the Zohar, the great book of Kabbalah, is literally "radiance". But the same letters give to beware and to warn: a warning is a light flashed in your face, a thing made to shine at you so you take care. So brightness becomes vigilance. Watch how the binyanim split the labour: the simple verb shines, the nif'al keeps oneself careful, the hif'il throws the warning at someone else. 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.
The split is the lesson here. Pa'al = the bare act of shining. Nif'al = the reflexive guard, keep yourself careful ("!הִזָּהֵר" = watch out!). Hif'il = the causative, make a warning land on someone. Pi'el, pu'al and huf'al are not lexicalized for this root.
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.