Stars and the gas between them are intrinsically coupled. Massive stars form in clouds of molecular gas and illuminate them with their radiation creating regions of ionized hydrogen (HII regions) and photodissociation regions (PDRs). In all but the closest star-forming regions, observations of "star formation" are actually observations of young massive stars. These are detected either directly (e.g. in the far ultraviolet continuum) or more commonly indirectly, in the form of reprocessed radiation from dust or gas at certain wavelengths. The emission line spectrum of HII regions and PDRs is a widely used diagnostics of gas properties such as density, temperature, pressure or metallicity. With a newly developed scheme that couples a stellar feedback model (warpfield) to a plasma simulation code (cloudy) and a radiative transfer code (polaris) we can now produce synthetic observations both of individual sites of star formation and of star-forming galaxies. The method helps us derive basic properties of star-forming regions given their nebular emission and we show how these regions evolve in various diagnostic diagrams (such as BPT-diagrams).