tondi

Two blades, not one. This is the distinction to take away. Tondi is what a pair of blades does, closing past each other: scissors, shears, clippers, secateurs. Tranĉi is what a single blade does, pressing through: a knife, a sword, a saw. Hair, sheep, hedges and grass are tondataj; bread, rope and meat are tranĉataj. English uses "cut" for all of it and gives you no help at all.

The payoff. Add -il-, the tool suffix, and you get tondilo - scissors. It is one of the cleanest derivations in the language, and once you have it you also have tondilego for garden shears, ungotondilo for nail clippers, and tondado for the act of shearing itself. A learner who has met tondi never has to look up "scissors".

A minimal pair worth the warning. Tondro is thunder. One letter apart, two unrelated roots, and both plausible in a sentence about a summer afternoon. Ni tondis la ŝafojn dum la tondro is a real sentence and a real trap. Say them apart deliberately: ton-di, ton-dro.