Elements normally cannot be broken down into simpler substances by ordinary chemical reactionsBut how can they?

By 'normally,' we mean 'through chemical means.' Strictly speaking, the atom (the smallest single unit of an identifiable element) is the smallest piece of something that there can be, within the field of chemistry, that is. The word atom is derived from a greek word that means 'indivisible,' because it is usually just that, indivisible.

Chemistry is largely a science of how atoms and the electrons orbiting atoms behave and interact. And that's why there is always this caveat; outside the field of chemistry, nuclear reactions can break apart individual atoms and recombine them. Nuclear reactions are differentiated from chemical ones because they go beyond mere electron interactions, into the nucleus of an atom, and break apart, rearrange, and recombine the internal constituent parts of atoms, protons and neutrons.

Examples could include the fusion of hydrogen into helium that powers the sun, uranium enrichment to create nuclear fuel, or radioactive decay in general. Only through nuclear reactions can one thing truly become something completely different. Within the field of chemistry, there is no 'different,' only temporary recombination.