I know a little about fenics and freefem.
There exists a big difference between those and OpenFoam?
They are used in a similar way (editing a file and writing code)? or perhaps it is made for other types of problems? Is it appropriate for solid-structure coupled problems?
Finally, is it better than the previous ones in some sense? (Or perhaps it is so different that this question does not make sense).
My question is to know if I spend time learning how to use it.
The fundamental difference between OpenFOAM and FreeFEM/FEniCs is the underlying approach in discretizing PDEs. OpenFOAM uses a finite volume method whereas FreeFEM and FEniCs employ the finite element method. Historically, finite volume methods were chosen for stability, but finite element-based CFD is what most research groups focus on (based solely on personal observation) because there's a lot you can play with to improve the method. For a more involved discussion on the differences between the two methods: differences between FVM and FEM.
Each of these codes are open source and have development as well as user communities. I'm sure someone has done Fluid-Structure Interaction (solid-structure coupling?) problems in OpenFOAM. FreeFEM advertises that capability on their website. Likewise, FEniCs can probably handle it as well.
I think you need to specify why you need this type of software.
