To + __ing (?!)

Let’s look at what we are trying to communicate in this sentence:

[…thing A…] is the key to [..thing B..].

OK. So thing A is obviously a thing or a behaviour that will help us to achieve thing B, which is a thing or a behaviour that is difficult to achieve without thing A. If thing B is a thing, then it is a noun and not a verb, so we need the noun form of 'grasp', which is 'grasping':

For some reason the expressions the key to and the answer to often lead to this error, so watch out for these! In this structure, to is a preposition and not part of the infinitive to + V1.