The structures "to... (afraid to do)" and "preposition + -ing (afraid of -ing)" both involve the emotion of fear expressed in different contexts:

The structures "to... (afraid to do)" and "preposition + -ing (afraid of -ing)" do indeed both express fear, but they are used in different contexts.

1. "Afraid to do": This structure is used when someone is scared of the possible outcome of an action that they might take. For example, "I am afraid to go outside because it is dark." Here, the speaker is scared of what might happen if they go outside in the dark.

2. "Afraid of -ing": This structure is used when someone is scared of a general situation or action, not necessarily one that they will undertake themselves. For example, "I am afraid of flying." Here, the speaker is scared of the act of flying itself, whether or not they are the one doing it.

So, while both structures involve fear, "afraid to do" is more about fear of potential personal actions and their outcomes, while "afraid of -ing" is more about fear of general situations or actions.