from Wikibooks: Lojban/Place structure
As stated before, Lojban is based on "predicate logic." What this means is that in Lojban, like in math, sentences (and functions) have "arguments," or places that you stick information. Lojban makes sense of these different arguments by their position in the sentence, called "Place Structure."
What’s in a sentence?
Take this sentence:
"I give you this."
What makes up this sentence? How does it convey information? The core of the sentence is "give", a word describing the relationship of how the other words are involved with each other, how they connect and relate. Those involved words, representing objects, are "I", "you" and "this". These could be said to be variable — they are not fixed with the relation "give". Rather than giving you "this" I could give you "that" instead. Or maybe it is you who gives me something, but whatever the related items, we would still be describing a relationship of giving. We could say that the relation "give" has three variable places that we can fill with different things. Thus we can define "give" as such:
Someone gives someone something.
We could label the variable places more clearly, numbering them and using "x" which is commonly used to describe something unknown or unspecified:
x1 [donor] gives x2 [receiver] x3 [gift]
or more simply:
x1 gives x2 x3
This says that we have a variable ("x") place 1 that is a donor, a place 2 that is a receiver and a place 3 that is a gift. They share the relation "give".
The Lojban word for "give" is dunda. It has this definition:
x1 [donor] gives/donates gift/present x3 to recipient/beneficiary x2 [without payment/exchange]
We can simplify this:
x1 gives x3 to x2
You might notice that the gift and receiver switched places here, which we represented in English by adding the preposition "to".
We call definitions like these "place structures", because they show the structure of the variable places we can fill. In Lojban, we call the relation a selbri and the variables sumti. We call the whole sentence a bridi. Linguists and logicians call the sentences "predicate relationships", the variables "arguments" and the relations "predicates" or "predicate relations".