This is a geminate root: the last two letters are the same ד, which makes its forms compress (you say לָמֹד, not the regular three-vowel shape). One idea runs through it: taking a measure. Measure a length and you get a size; measure the economy and you get an index; a middah is even a measure of character. Measuring yourself against an obstacle becomes coping with it. Each word below is a different thing being measured, 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.
Because the root is geminate (מ-ד-ד), the pa'al לָמֹד drops to a tight two-syllable shape instead of the usual לִקְטֹל. The nif'al לְהִמָּדֵד is the plain passive ("to be measured"); the reflexive flavour of measuring-against shows up not as a verb but in the noun הִתְמוֹדְדוּת below.
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.