Understanding Modal Verbs
In the sentence "You have trusted him so much and given him access to your private files," the choice of the modal verb depends on the context of the statement. Let's explore the options:

1. Ought Not To
- This phrase indicates a recommendation against an action.
- Example: "You ought not to have trusted him so much."
- It implies that trusting him was a mistake based on moral grounds.

2. Need Not
- This phrase suggests that there is no obligation to do something.
- Example: "You need not worry about trusting him."
- It conveys that there is no necessity for concern.

3. Must Not
- This phrase expresses a strong prohibition.
- Example: "You must not give him access to your private files."
- It indicates that it is imperative to avoid this action for safety.

4. Dare Not
- This phrase indicates a lack of courage or fear regarding an action.
- Example: "You dare not trust him with your private files."
- It reflects apprehension about the consequences of trusting him.

Conclusion
The correct choice depends on the intended meaning:
- If you want to emphasize that trusting him was a mistake, use "ought not to."
- If you want to indicate there’s no obligation to worry, use "need not."
- For a strong warning against giving access, use "must not."
- To highlight fear or doubt in trusting him, use "dare not."
Each modal verb carries a different weight, so choose according to the message you wish to convey.