I'm following a tutoriel. Everything went right until suddenly, when I draw a circle or a square, all I get is the dotted outside line (the one with the two little squares and the little circle), although I have selected a colour. What happened ?

At a guess, you've got transparency set somewhere, either in the colour's alpha value, or on the object itself. See the attached screenshot. Make sure both these values are set to 100. Check the layer transparency too, just in case that's been set to zero.

There's also a tutorial, if you need more help: https://inkscape.org/forums/tutorials/what-to-do-when-a-tool-stops-working/

Thanks a lot z3z and Maren Hachmann. I got it !

I hate to bother you, folks, but I went a little further in the 2nd tutorial and how I'm supposed to cut off a form from another form (in this case the shape of an apple tree from a square). 
As instructed, I selected the form to be taken off, then the from which it whould be taken, chose Path and then Difference. Nothing happened. So I tried to select the square first, then the apple tree. Nothing happened. Then I tried to do both, using "Control -". Nothing worked.

What do I do wrong ?
Can you help me me this time as well ?