[lojban-beginners] A problem with cu

[lojban@65536.org]

coi rodo (or is simply {coi} better?)

After some Lojban lessons I have still some problems with {cu}. As far
as I understand it {cu} indicates that the following word is a selbri.
It separate two gismu: 

{le nanmu cu klama}

OK, but why do you use {cu} in the following sentence:

{lenu mi cilre fi la lojban. cu xamgu mi}

{xamgu} is a selbri, so you can use {cu}, but is

{lenu mi cilre fi la lojban. xamgu mi}

correct? {la lojban.} is a cmeme - there is no need for a {cu}?
Another {cu}-problem:

{la daucac. tcika lenu mi klama}

is OK. If you want to use {se} it becomes something like

{lenu mi klama cu se tcika la daucac.}

Why is {se} not enough to separate {klama} and {tcika}?

mu'o mi'e .iens.gutsait. (or is mi'e ... enough? What is the
difference?)