In this presentation, I will show how the analysis of the spatial distribution of young stars (YSO) and its comparison to the core population can reveal stellar formation episodes in star forming regions, and help us understand the fragmentation process. I will focus in particular on two very different regions: the relatively massive cluster NGC2264 and the Taurus association. Our recent study of the clump and YSO populations in NGC 2264 suggests a sequential star formation, starting from the northern part to the southern subregion. We propose also that the observed mass segregation in the central part is inherited from that of clumps, itself resulting from a dynamical scenario of star formation. In contrast, the Taurus association is much less dense and dynamic, which allowed us to identify pristine small scale stellar structures, called NESTs, that correspond to the preferred sites of star formation. The analysis of their properties indicates two distinct regimes which may correspond to the fragmentation of a single core or a group of cores. In addition, a further study at even smaller scale argue for a cascade fragmentation scenario. I will then finish by presenting the public sofware called S2D2 that we developed in the framework of the StarFormMapper project for the community to look for NESTs in star forming regions.