Milord

[Janek]

Salut!
Pourquoi il y a une forme "vous asseoir à ma table" dans les paroles de chanson de Édith Piaf? Devrait on être "asseyez"? Pardon my french. ;)

[Vedas]

And in Belgium they don't make the distinction between "savoir" and "pouvoir" (so just like the verb "can")

est ce que tu saurais m'aider?= est ce que tu pourrais m'aider?

[Sandra]

I can say that generally, a negative without "pas" is used in a more formal/ historical context.

"Puis" is an old present form of "pouvoir" where you rarely add the "pas", with the normal form "peux" you have the choice to add the "pas" or not, same with "savoir" as George said.  I'm sorry there is no real rule, it's only a matter of  "language type" I would say :

"Je ne puis trouver d'excuses..."  -> Very old fashioned, elevated language, almost never used

"Je ne peux trouver d'excuses..."  -> Formal and educated language

"Je ne peux pas trouver d'excuses..."  -> Standard/everyday speech

"J'peux pas trouver d'excuses..." ->  this type of contraction is a very bad way of speaking. (a lot of young people unfortunately use it today), you may hear it but it's NEVER appropriate to write anything.