When all you have is a hammer, everything looks like a nail.
More French Proverbs:
An old dog does not bark for nothing.French
There is nothing so well done but may be mended.
French
God alone understands fools.
French
Patience is bitter, but it bears sweet fruit.
French
They will be hushed by a good deed who laugh at a wise speech.
French
Flying birds have no master.
French