I think this is lazy on several levels. First, it is a trivial task, and the least you could have done is show us the command that didn't work for you. Second, it would be nice to know how do you want the files to be formatted (still comma-separated? or not? does .maf or .csv come first when pasting?) and the resulting file names after pasting. It also would be nice to know what shell you are using. As it is, we have to guess these things on our own.

I think this post, along with other posts from one EU members make me rethink forum's future path. I think it's the time to say bye to this forum.

Dear Mensur, thank you very much for your answer, you have been very kind and generous. It was a great help. I apologize for the omissions, all observations are correct, sorry.

This was my initial script

for i in $(ls *.maf)
do
  for j in $(ls *.csv)
  do
    paste $i $j > $i.M.csv
  done
done

But this always added the same csv file to all *.maf.

With your suggestion I changed it like this

for i in $(ls *.maf)
do
  for j in $(ls ${i%.maf}.csv)
  do 
    paste $i $j > $i.N.csv
  done
done

Your script also worked fine

Thank you very much and sorry for the inconvenience.