Measurement determines the observed nature
An electron behaves like a particle when you detect where it is, leaving a discrete mark on a detector screen. The same electron behaves like a wave when it passes through two slits and creates an interference pattern characteristic of waves. This is not a limitation of our instruments or knowledge but an irreducible feature of quantum reality: the electron does not have a definite 'true' nature independent of how we observe it. If you set up detectors to see which slit it goes through, the interference vanishes and you see two particle-like bands. If you remove the detectors and let it propagate freely, interference reappears. The act of measurement forces the system to 'choose' a behavior.
Momentum and position are fundamentally linked
Wave-particle duality is encoded in the uncertainty principle: the more precisely you know a particle's position, the less precisely you can know its momentum, and vice versa. A particle confined to a narrow slit must have a spread in wavelength (hence momentum) to form a localized wave packet. A pure plane wave, by contrast, has a single sharp momentum but extends everywhere in space. Every quantum object carries both wave and particle aspects simultaneously, and which dominates in your observations depends on experimental design.