I would like to know whether there is any solution to avoid the stiffness and crispness of bread of oven baked sandwiches which uses normal white bread as the base. I used to make sandwiches with bread and always these bread after baking turns out really crispy and stiff and sometimes it get burnt. I butter the bread sometimes and sometimes not. Either way I end up with crispy bread sandwich which is difficult to bite and affects our mouth skin. How can we make the bread base of the sandwich soft?
Wrap the sandwich in foil before baking so the moisture stays in the bread. That will prevent it crisping up.
The bread gets crispy and "stiff" because it dries up completely.
A good toaster should toast a slice of bread quickly so that the exterior is toasted and the interior barely hot; a bad toaster will not be warm enough and will dry up the slice of bread.
Same thing when you do an oven baked sandwich, it should be done on high heat so that the bread toasts up and crisps up on the exterior while not drying up the bread.
You say "toasted" in your title but there are various ways to achieve this, different ines dominating in different countries. If you have a slot toaster (common in the UK, much less so in the rest of Europe, and apparently common in the US/Canada) there are toastabags (although this link is lakeland, they're on e.g. amazon as well). These are reusable bags made of non-stick sheet, open on one edge. You make the sandwich, put it in, than stuff it into the toaster. I tend to give the sandwich 20 seconds in the microwave before toasting to make sure the cheese is nicely melted.
Alternatively if your oven has a grill setting that will work a lot better than oven mode.