unergative verbs and unacussative verbs

Hello, people

As you see I have some difficulty in understanding classes of verbs, especially if they are connected to one another.
The classes of unaccusative verbs and unergative verbs are a bit confusing to me.
According to thier definition, unergative verbs are monadic verbs which have 1 external argument, whereas unaccusative verbs are monadic verbs which have 1 internal argument.
the examples given are:
wilt, fall, collapse, burst, vanish, rust, fail, expire - unaccusative V
run, caugh, laugh, fly, sing, yawn, swim - unergative V

can someone please explain to me how come the unergative class has an external argument and the unaccusative class has an internal argument?

you can use the examples given.
thank you very very much.

Generally speaking, the subject of a sentence, where the subject is the agent of the action of the sentence, is the "external argument".  Other noun phrase arguments of the sentences (particularly the direct object, also called the accusative) are referred to as the "internal arguments".

According to thier definition, unergative verbs are monadic verbs which have 1 external argument, whereas unaccusative verbs are monadic verbs which have 1 internal argument. the examples given are: wilt, fall, collapse, burst, vanish, rust, fail, expire - unaccusative V run, caugh, laugh, fly, sing, yawn, swim - unergative V

So it seems that, according to the definitions you gave, the unaccusatives are missing a subject and the unergatives are missing an accusative -- in some sense -- so that the underlying structure of the concepts of wilt, fall, run, laugh, etc., are something like:

... wilts me, ... falls me, ... collapses me, ... vanishes me, ...
I run, I laugh, I yawn, ...

Thus, I imagine the central claim is that wilting, falling, collapsing, etc. are things that happen to me; I do not actually do them.
Conversely, running, laughing, yawning, etc. are things that I do (as an agent); they do not just happen to me.

But I'm merely speculating.  None of it has never made a lot of sense to me, and over the years I've found what seem to me to be contradictory definitions of these verb classes, so let me know if you find out what it all means.  I'd be interested in finding out. 

CJ