Taking an onomasiological approach and a dynamic usage-based perspective, this study explores how four beginning L2 learners of Finnish develop in expressing existentiality (‘there is something somewhere’) before and after instruction. Data were collected weekly over a period of nine months and examined for conventionalized and non-conventionalized constructions that express existentiality. As expected from a dynamic usage-based perspective, both inter-individual variation and intra-individual variability were identified. The initial repertoires of two of the learners were quite variable, as they used several different non-conventionalized constructions before settling on more conventionalized ones. In contrast, the two other learners did not independently try out different ways of expressing the targeted meaning but started to use the conventionalized Finnish existential construction only after pedagogical intervention. As one would expect from a usage-based perspective, some learners’ initial repertoires included some item-based constructions that were similar to each other. As far as instruction is concerned, for all learners there was an increase in the use of the conventionalized construction after an explicit intervention, but the use was not morphologically accurate. The findings confirm two commonly held hypotheses in dynamic systems approaches: Learners own their own learning trajectories and initial trajectories are sometimes characterized by high degrees of variability because learners need to try out different strategies before they can adapt to the requirements of the new situation.