For example, point 2 asks to translate:
"The pack of three cats bite the dog",
which I translated as
{lu'o le ci mlatu cu batcy le gerku},
The selbri should be {batci}. :)
NB: no gismu can contain the letter .y.
Apart of that Your translation is alright, though it's probably better to say
{lu'o lo ci mlatu li'o}
according to the accepted proposal/convention called "xorlo" (which has
been mentioned by Antonio).
but checking the answers it proposes
{lei ci mlatu cu batci le gerku}
li'a This is pretty much the same as the above.
or
{lu'o ci le mlatu cu batci le gerku}
Again {lo} would be more appropriate.
[...] If in English I said "the [pack of] three cats ..." am I not speaking of
three cats I have in mind, otherwise I would have said "a [pack of] three
cats ...". Am I wrong.
Right, le velcli (Lojban for Beginners) uses the older convention concerning
the gadri {le}: this says that {le broda} ought to be translated as "the thing(s)
*I call* broda", whereas xorlo declares that {le broda} is (are) the concrete
broda('s) the speaker has in mind, as opposed to {lo broda} which means
any of the actual broda's.
Hope all is clear now and I'm not nalsmu cusku. :)