I recommend reading the earlier parts first. (License is CC BY 4.0.)

To enable more natural-looking sentences, let's give Reverse Polish some loanwords: nouns will introduce their concept to the stack, and adjectives will modify the top item, as will "words" like time-of.

When we say a quote, we mean to evoke the meaning of that quote. It essentially means "take the meaning of the top item and apply it as though it happened". More briefly, it means "this (top item) applies". We can take this a step further, so that saying anything commits the speaker to its validity. Because of this interpretation, I'll start writing say as . instead. The effect still pops the evoked item off the stack, however.

The sentence bear black ., for instance, might mean "There is a black bear.". A fancier example is girlfriend my copy . Canadian ., meaning "My girlfriend (who is real) is Canadian."

Conversations are in some sense an exchange of ideas. As such, the conceptual stack is shared between the participants. Questions become a matter of leaving a concept on the stack, like a sentence to finish.

Before we go into the first sort of dialogue in the language, let's add one more word, the, which refers to the most recently mentioned (or most relevant if unmentioned) concept described with the effect of the following word.

A sample exchange is

1: the party time-of
   "When's the party?" (lit. "Time of the party?")
2: tonight = .
   "It's tonight."
1: the party formality-of formal =
   "Is it formal?" (lit. "Truth of the formality of the party equaling formal?")
2: not .
   "It's not."

If we don't want a pause to be taken as a question, we can mark questions with tone or add a filler word like uh. Some more words we could add are pronouns like that to refer to the last concept that was .ed (so we don't have to copy things in advance all the time).

I think that'll do for the main parts of the series. The rest will be writing the appendices on assignment semantics and the comparison to concatenative programming languages (so you can see why I decided on the system in part 3). Beyond that, posts will most likely actually be about my language si ka, as I finally have a reference to point people to when the grammar gets too unusual.

If you liked reading these, have suggestions for other topics, or didn't understand something, feel free to comment here, and thank you for reading!