"The boys like that."
Translation:Is maith leis na buachaillí é sin.
I wish Duolingo gave good enough grammar explanations that people like you weren't forced to crowd-source their grammar answers. Daltaí is, at least, better than what you're likely to get on these forums.
The difference is not between leis (which is simply the form of le used before an and na) and le. It is between the initial mutation or lack of it used after an and after na. In your examples, there is an initual mutation after an but not after na. That pretty much sums it up. Mutate after an but not after na. Charts: http://www.nualeargais.ie/gnag/artikel.htm#praeposition
Note that there are two accepted systems of initial mutations after prepositions. I would say ar an bhus and leis an bhó, never ar an mbus or leis an mbó, and I would be right, too. Dunno if Duolingo tells you that.
More than you ever wanted to know about prepositions: http://www.nualeargais.ie/gnag/praepos.htm
Unsolicited advice: Get a grammar book!
Thank you very very much! I actually have a grammar book (Christmas MacCongáil) and searched and searched, but you're right, it's on p 22. I just assumed an and na were the same thing. But Mr Christmas is clear about it! The Ulster - Munster rule that you mention appears passively in Duolingo and as a rule they allow both, I think. Thanks !