ことができる

ことができる means "can" or "be able to". It follows a verb in the dictionary form, and できる at the end takes the tense and the politeness.

It also covers what is possible or allowed somewhere, not only what a person is able to do.

ことができる or the potential form

The potential form (はなせる話せる, よめる読める) says the same thing in one word. In conversation it is the usual choice; ことができる sounds more formal and turns up in writing, announcements and explanations.

The two do not combine. The verb before こと is the plain dictionary form, never the potential.

• × 話せることができる → 話すことができる / 話せる

The object keeps を with ことができる, since できる attaches to こと rather than to the verb. Only the potential form lets を change to が.

• × 日本語が話すことができる → 日本語を話すことができる

Usage notes

These go beyond a verb before ことができる.

A noun with できる

A noun takes できる directly, with が and no こと: にほんご日本語ができる means "can speak Japanese", てにすテニスができる "can play tennis".

する-verbs

For a する-verb, the potential form is the noun plus できる, so 運転することができる and うんてん運転できる say the same thing; the second is shorter and more everyday.