Il ne mange des pâtes que le samedi. He eats pasta only on Saturdays.
Not sure the English is quite right there.. shouldn't it be he only eats pasta on Saturday.. he eats pasta only means he eats nothing else
l ne mange que des pâtes le samedi. He eats only pasta.... That is correct and is synonymous with he eats only
The French looks very clear.. it is the English that is impossible!