〜ことができる

〜ことができる is the periphrastic potential — "is able to do X." 日本語を話すことができる = "I can speak Japanese." It coexists with the morphological potential (the 〜られる / godan 〜える forms): 日本語が話せる expresses the same meaning more compactly.

When to prefer which:
• 〜ことができる is more formal and more emphatic about the ability itself. Common in written Japanese, instruction manuals, signs.
• Morphological potential (話せる, 食べられる) is shorter, more conversational, more idiomatic in speech.

A subtle difference: 〜ことができる accepts almost any verb cleanly. Morphological potential has irregular cases (する → できる, 来る → 来られる/来れる, 行く → 行ける) and a register where ら-drop forms (食べれる, 見れる) are still controversial in formal writing.

Note the change in particle on the object: with 〜ことができる, the object takes を (the embedded verb's argument). With morphological potential, the object often shifts to が — 寿司が食べられる — though を is increasingly common in modern speech.