As usual, I need to ask for more info...
What source file type are you working on?
I have two segments, first of which can not be translated at all (it's a one word translatable only through changing the form of the next segment's words).
What do you mean by this? Can you explain?
I need to expand this segment, but TagEditor HELP topics don't provide enough information.
I have tried to [Ctrl]+[Alt]+[Page Down] - nothing happens. (What exactly should I select before trying to expand - 1st segment? open or closed? etc?)
First segment needs to be open, then you can expand - unless this is prohibited by the document structure. Are there breaks or specific tags between the two?
Also note that, in order to expand, the second segment must be untranslated - you cannot expand into a translated segment.
It looks like there is no option to be set in Setup>Segmetation rules to do this.
Depending on the type of file you're working on, segmentation is predominantly determined by the DTD settings.