When does an opponent's tile (dark red) become flippable (light red)? What conditions will turn an opponent's "safe" tile to "unsafe" so that I can steal that tile?
A tile which you control can be defended (to use the official term) by controlling all 4 (or 3 or 2 along edges or corners respectively) tiles immediately adjacent to it. (Above, below, and to either side. Diagonals don't count).
A tile which has been defended can be used to complete words, but control of it will not change when it is used in this manner. If the adjacent tiles are flipped, the defended tile will no longer be blocked, however, this does not happen simultaneously. In other words, it takes at least two turns to flip a defended tile, no matter what.