Extend typically implies length. Expand typically implies all directions.
You can extend time, but you wouldn't expand it.
You can expand -and- extend a building.
Liquids expand when they freeze, but they don't extend.
As for extend and expand as adjectives, there's no real 'rule' to them. They are determined by context.
Though, in general, 'expand' has a larger impact in meaning that 'extend'. Compare a DVD release offering "expanded" or "extended" features. Expanded makes you think they are not only extended, but somehow enhanced in some other way.
tl;dr: extend = to lengthen. expand = to spread out.