Bravo, Miyeong!
I think one thing we can learn from Halliday is to treat ALL utterances as multi-functional. All utterances do many things at the same time.
I don't know why this is so hard to grasp, but it is--even for native speakers. We assume that whenever someone speaks they really have only ONE thing in mind.
But of course that's not true: when the teacher greets the class, the teacher has in mind:
a) This is how people say "hello" in English. (The textual function)
b) It's time to start the lesson. (The ideational/experiential function)
c) Please be quiet and pay attention. (The interpersonal function)
Now, the reason why the "mood" is hard to find in our target sentence is because it's an IMPERATIVE. An imperative often consists of a SINGLE word:
Look!
Listen!
Stop!
But we can still find all three functions; they are just FUSED, or COMBINED in a single word!
(You) Look!
(Theme) Rheme
(You) Listen!
(Actor) Process-Mental
(You) Listen!
(Mood) Residue
Suppose you want to take this further. Where is the predicator? What about the subject? What is the finite "fused" to and "combined" with? Is it fused with the subject or with the predicator?
Why is the MOOD more important to the interpersonal function than the predicator? Why is it LESS explicit (in the case of an imperative)?
This gets us back to our first question. Is a SHORT sentence really an easy one? Is it easier to understand one word? It certainly is NOT easier to analyze it!
dk