"BR1IN is unable to rout/retreat/fall back and is forced to surrender!" - This means a unit that had to rout/retreat/fall back had no valid direction in which to move.
"BR1IN is exhausted, and surrenders!" - This means a unit lost a melee and had fatigue of 16 or greater.
"BR1IN is trapped by enemy forces and surrenders!" - This means that a unit lost a melee but was so entangled with the enemy units that it was unable to leave its present tactical square, even if it had a valid fall back direction available. The odds of this happening are higher if the unit had a flank turned (higher still if both flanks are turned) but it can happen even if not. This message is printed only for units falling back, not for those that failed morale checks and are retreating or routing.
"BR1IN tries to rout/retreat/fall back but is blocked, and surrenders!" - This means a unit tried to rout/retreat/fall back after combat, but all the squares to which it could legitimately move were impassible terrain, or contained enemy or neutral units, or in the case of rout, were invalid strategic moves.
"BR1IN has no direction to retreat and is forced to surrender!" - This
means that all valid directions in which a unit could retreat were blocked
by hostile units.