Stars form predominantly in groups which display a broad spectrum of masses, sizes, and other properties. Despite this diversity there exist an underlying structure that can constrain cluster formation theories. We show how combining observations with simulations allows us to disclose this underlying structure. One example is the mass-radius relation for young embedded associations which follows ${M_c} = CR_c^\gamma $ with γ = 1.7 ± 0.2.0.2, which is directly related to the mass-radius relation of clumps. Results based on GAIA DR2 have demonstrated that young stellar groups (1–5 Myr) expand and that this expansion process is largely over by an age of 10–20 Myr. Such a behaviour is expected within the gas expulsion scenario. However, the effect of gas expulsion depends strongly on the SFE, the gas expulsion time scale, etc. Here it is demonstrated how existing and upcoming data are able to constrain these parameters and correspondingly the underlying models.