Mondeo Mk4 2005 2.0 Tdci Gear' S Problem
That shouldn't be the case. The Mk IV started in 2007. Anyway, I assume that it is a normal, 'H-patter', gearbox with six forward gears?
Sometimes it takes two three tries to select but sometimes it works fine, other gears work fine as well?
What happens when it doesn't go in? The gear lever doesn't goes as far as normal, but is then blocked?
Is it the case that the clutch travel is normal, or does the clutch only work at the bottom of its travel. If the issue is that the clutch doesn't fully disengage when depressed (either it only works at the bottom of the range of travel, or the car tends to creep when the clutch is down, but the car is in first/reverse on level ground), then the problem is probably the clutch (or initial stages of a DMF problem) rather than the gearbox.
Assuming that the problem isn't the clutch, then the gearbox oil is the next thing to suspect; probably the car has done a few miles by now, so changing the gearbox oil isn't a bad idea, anyway.
<Edit: Usually, it is the case that 'bad gearbox oil' is worse at start up, in winter, so if it is worse then, you probably want to change the oil.>
Gear linkage is a possibility, but usually gears 'wander' around a bit, if a linkage has gone bad. It is always (imho) worth lubricating the gerabox mechanism with, eg, bicycle chain lubricant, as this sometimes helps the feel of the shift, but that isn't usually enough to make blocking gears work again.
Have you, or your brother, tried double de-clutched changes from second to first at low speed? Does that work?