Hello Gérald,
There are two possible solutions to combine two or more curves:
You can use function "Create a Contour by manual selection":
(doubleclick the last curve to merge).
The advantage of this function ist, that it automatically trims the curves end to end.
It creates a path object as the result.
the other way is to put all curves you want to merge in a group and activate the formfinder for this group.
to select connected curves/lines, you can use the
"Select connected objects (path)" function
And then press the group or folder button.
for activating the formfinder, you can use the object editor or simply press [F] with the mouse over the group symbol in the tree.