I'm an A2 igsce math student and I'm taking mechanics for the first time in math this session.
I know that what I'm going to say is wrong judging from my teacher's reaction when I asked him about this but unfortunately he didn't explain to me why it's wrong.
So, when we draw two forces on an object, we usually assume that they are applied at the same time but do we also assume that they were applied for a certain duration of time, like one second maybe?
I mean, how are we supposed to get the magnitude of the resultant of these two forces meanwhile we don't know the duration? To make my point clearer, for example, couldn't there be more than one force in the exact direction of the resultant force that would make our object/point reach the final speed it is supposed to reach after these two forces are applied for a certain time? — thinking about it now the examiner never mentions for how long two forces should be applied, but anyway.
But each force would need a certain amount of time, like a weaker force will need more time while a stronger one will take less time and so on. And all these forces are in the same direction of the resultant force of course.
At this point in your course it appears that all forces that are treated are constant in time and infinitely present like gravity, a steady wind, or friction (there the force is not constant, but the cause of it, or friction coefficient, is).
You might be thinking more like of temporary forces like a rocket engine (or more mundane, a car engine) or changing forces like the magnetism of an electro-magnet under alternating current.
A force only provides acceleration (actually, changes in impulse) as long as it is "switched on", two forces can only be added in the intended sense if they are "switched on" at the same time. The confusion of your teacher could be because the forces discussed so far are "constantly on".