I have wondered on occasion about the distinction between these two words.

Problem:
These two adjectives are not quite synonyms.

Explanation:
The primary definitions of the adjectives “capable” and “able” speak to the same concept: having adequate power to do something.

The primary definition of “able” is having the necessary power: “The truck is able to pull a two-ton load.”

The primary definition of “capable” is having power; competent: “She was a capable museum lecturer.”

Here is the distinction between these two adjectives:

• “able” refers to having power equal to the effort required;
• “capable” refers to having power to meet ordinary requirements.

Solution:
Favor using “capable” when referring specifically to ordinary requirements; otherwise, use “able” instead.