The business of a selbri (as you hopefully remember from Lesson 2) is to point out a relationship between one or more things (its sumti.) So when you say dunda, you mean that there's a giver, a receiver, and a gift involved. When you say klama, you mean that there's a traveller, a destination, an origin, a route, and a means of transportation involved. When you say mensi, you mean that there's someone who is a sister, and someone that she's a sister of. And so on.
Now, when we put an article in front of a selbri, we turn it into a sumti. But the selbri within a sumti remains a selbri: it still indicates that there's a relationship between some sumti of its own. If you say le dunda, you still mean that there is something the 'donor' is giving, and someone they are giving it to. If you say lo xe klama, you still mean that there is someone going in the 'vehicle', somewhere they are going to, somewhere they are coming from, and some route they are taking. And as we've already hinted, it is meaningless just to say le mensi, just as we don't say the sister in English: a sister is always a sister of someone.
Previously, we have used pe to attach sumti to other sumti, in order to narrow things down. But that doesn't necessarily mean that what follows pe is a sumti of what comes before it. So if I describe my sister as le mensi pe mi 'my sister', for example, that might be the same as saying zo'e (= my sister) mensi mi. But if I say le jdini pe mi 'my money', I certainly do not mean zo'e jdini mi — that I am the mint which issued the money! Obviously pe won't do as a general solution to filling in the selbri you might need.
If you have a selbri contained inside a sumti, the way to give it a sumti of its own (an internal sumti) is to add it in with be. You'll remember (we hope!) that, when a selbri gets an article, its meaning is the x1 place of that selbri. By default, be fills in the x2 place of the sumti. So:
la renas. mensi mi
Rena is my sister
le mensi be mi
la renas. te dunda le cukta
Rena is given the book
le te dunda be le cukta
The recipient of the book
As you can see, be can translate — often but not always — to English of. In fact, it covers surprisingly many of the functions of of. And because it is tied to a specific place of the sumti, its relation to the main sumti is unambiguous (another one of Lojban's 'selling points'!)
la renas. klama la sidnis.
Rena is going to Sydney
le klama be la sidnis.
The one going to Sydney
What do these sumti mean in English?