Softwood, Hardwood, and What Actually Matters
Softwood and hardwood are botanical terms, not descriptions of hardness. Hardwoods come from broadleaved trees that drop their seed enclosed; softwoods come from conifers with needles and cones. The classification says nothing reliable about density, and there are well-known counterexamples in both directions: balsa is a hardwood and is among the softest timbers in commercial use, while several conifers are harder than a number of common hardwoods.
What actually matters for furniture is a set of independent properties that have to be considered separately. Density governs how a timber takes an edge, how it wears, and how heavy the finished piece will be. Hardness at the surface governs how it resists denting in service. Stability governs how much it moves with the seasons. Workability governs how it behaves under a plane. A timber can rank high on one and low on another.
Grain structure matters at least as much as density, because it determines how the surface will finish. Ring-porous timbers such as oak and ash have a coarse open texture that takes a filled or a cerused treatment well and will never look glassy under a thin finish. Diffuse-porous timbers such as maple, cherry and birch have a fine even texture that will take a very smooth finish but which shows blotching under some stains, particularly the ones that penetrate unevenly into the tighter areas.
Movement varies a great deal between species and is the property most often overlooked at the timber yard. Two boards of similar weight and similar cost can differ substantially in how much they change width between a damp summer and a dry heated winter, and a wide panel in a high-movement species needs more allowance built into its construction than the same panel in a stable one.
There is also the question of what a timber does over time. Cherry darkens considerably with light exposure, so a piece that looks pale when finished will not stay that way, and anything left sitting on it during the first months will leave a permanent lighter patch. Oak is less dramatic but still shifts. Planning around this is easier than being surprised by it.
The practical approach is to stop asking whether a timber is a hardwood and start asking four questions: how does it work, how does it move, how will it finish, and what will it look like in ten years. The answers are species by species, and the botanical category predicts almost none of them.