He opened the letter with (shaking, shaken) fingers.

In the sentence "He opened the letter with shaking fingers," the correct choice is shaking. Here is a detailed breakdown of the grammar and usage:

• Grammatical Function (Present Participle): The word "shaking" is a present participle used as an adjective (attributive position). In English, present participles (-ing) describe an active state or a characteristic of the noun they modify. Since the fingers themselves are performing the action of trembling at that moment, "shaking" is the appropriate term.
• Active vs. Passive (Shaking vs. Shaken):
  • Shaking describes something that is currently in motion or vibrating (e.g., "shaking hands," "a shaking voice").
  • Shaken is a past participle and usually describes a state resulting from an external force or an emotional shock. While you can say a person is "shaken" (meaning they are upset or jarred), you do not usually describe fingers as "shaken" unless they have been physically moved by someone else.
• Semantic Context: When describing physical trembling due to fear, anxiety, or excitement, the -ing form is standard. You will find similar usage in phrases like "trembling hands" or "quivering lips".
• Collocation: In literature and formal writing, "shaking fingers" or "shaking hands" is a standard collocation used to convey nervousness. For more examples of how "shake" and its forms are used, you can refer to the Oxford Learner's Dictionary.

Summary of the rule: Use shaking to describe the fingers' own movement; use shaken to describe a person's emotional state after a shock. Would you like to see more examples of how present and past participles differ when describing physical reactions?