If you're a beginner learning from lojban.io, la karda or the wave lessons then when starting out, you will be used to Lojban sentences that look like this:
do dunda lo plise ku lo mlatu ku
You give an apple to a cat.
However, you would most likely come across sentences like this when reading discord chats in Lojban:
do dunda lo plise lo mlatu
Given lojban.io's description of lo's grammar, what would this mean?
Okay, so now we know how to use prenu as a selbri, and this lets us express sentences such as "I am a person" (mi prenu). But how can we say "I am talking to a person"? For that, we would need prenu to act as a sumti.
This is achieved by using the words lo and ku, as in the following examples. [...]
You simply place a selbri between these two words, and it takes anything that can fill the x1 of that selbri and turns it into a sumti. So while prenu means "is a person", lo prenu ku means "someone who is a person", or simply a person / the person.
The description sumti phrase after dunda appears to contain plise lo mlatu. Because there's no ku after plise, it appears to mean "an apple of strain a cat". This is obviously incorrect. The two sentences are in fact equivalent.
The word ku along with kei in the early lessons of lojban.io are terminators, words used to close off a grammatical construct like a spoken closing bracket. These words are essential to Lojban's ability to be unambiguous, with many Lojban translations of multiple interpretations of ambiguous English sentences differing only by their terminators.
Unfortunately, the resources mentioned at the start (which I will now call terminator-first resources) first introduce these words in an oversimplified, unhelpful way. lo ... ku and su'u ... kei are treated like a circumfix with both parts being equally important. It is perfectly understandable, but incorrect, to conclude from these materials that terminators are mandatory and that a phrase such as lo tavla mi do means "a talker to me about you".
Terminator-first resources have a chapter introducing terminator elision in which they explain the rules for dropping terminators. In the wave lessons and lojban.io, this chapter also introduces vau, a terminator that is missing from the early lessons because it's almost never used, but ku is also almost never used either yet it's taught as if it's an equally necessary part of forming a description sumti as lo until the terminator elision chapter!
All terminators can and should be taught similar to how terminator-first resources skip vau early on, and this is what Lojban For Beginners does. By teaching the word cu straight away, even the simplest example sentences will match those used by expert Lojbanists, saving a lot of confusion caused by incorrect (but understandable) assumptions and letting beginners just get talking straight away.
With the word cu introduced early on, no simple Lojban sentences (pronouns + basic description sumti) require terminators. They should be introduced when nested clauses or abstractors are taught, since those are the first grammatical structures a beginner will learn where their expressive ability depend on proper placement of terminators.
I think that terminator-first resources are unhelpful not just because they are confusing and misleading, but also because they frame terminator elision the wrong way. I do not think of Lojban sentences as containing dozens of terminators that I may drop if I'm careful.
When using Lojban, I'm conscious of when two grammatical structures will merge or nest improperly without a terminator and I only use a terminator (or cu) in those situations. The rest of the time, I do not think about all the terminators I'm skipping. I almost never imagine a ku and then mentally delete it after description sumti, as terminator-first resources seem to imply. I especially don't think of complex sentences as having a bunch of vau kei ku vau kei ku vau that I'm getting to skip with the following sentence marker .i.
You will be faster and more confident producing Lojban if you think of terminators as elided by default and included only to close off structures early. That is their real purpose. When used in their rightmost (default) position, they are meaningless and distracting.
Lojban's designers seem to have tried to make terminators unnecessary wherever possible. It doesn't make sense to teach Lojban in a way that ignores this. For example, if we really did want to nest a description sumti into another description sumti's place structure as in the wrong interpretation of lo plise lo mlatu earlier, this is achieved with a word be:
lo tavla be mi - a talker to me
Then the word bei allows you to put even more arguments in:
lo tavla be mi bei do bei la .lojban. - a talker to me about you in Lojban
This is the thing that gets special marking for a reason. If lo tavla mi do la .lojban. meant the same thing, ku would be necessary almost everywhere because it's much more common for nouns next to one another to be in different positional slots of the verb, rather than be nested into one another.
Elidable terminators are a distinctive feature of Lojban and many other (but not all) logical languages. Teaching them correctly is important. It's my view that terminator-first resources obfuscate Lojban's elegance, build wrong intuition and don't reflect real-world usage. Even after they teach elision, they frame it in an incorrect way that slows learners down in producing Lojban confidently.