Understanding the Correct Option: "Could"
When analyzing the sentence "When Juan was two, he __________ already speak very well," the correct choice is option 'b) could.' Let's explore why this option fits best.

Context of Ability
- The sentence discusses Juan's ability to speak at the age of two.
- "Could" indicates a past ability, suggesting that Juan had the capability to speak well when he was two years old.

Explanation of Other Options
- a) might
- "Might" implies possibility but does not confirm ability. It would suggest uncertainty, which is not suitable in this context.
- c) can
- "Can" denotes present ability. Since the sentence refers to a past time ("when Juan was two"), "can" is not appropriate.
- d) should
- "Should" implies obligation or expectation. It does not relate to ability and is irrelevant in this context.

Conclusion
- The use of "could" effectively communicates Juan's past ability to speak well at age two.
- Understanding the nuances of modal verbs (like could, might, can, and should) is essential for accurately conveying past abilities, possibilities, and obligations in English.
By selecting "could," the sentence accurately reflects Juan's speaking capability during his early childhood.