Ability to do, can do

This article shows two ways to express ability to do something in Japanese; using dekimasu and using the potential form of a verb.

Dekimasu

One way of saying that someone can do something is to use the verb "dekiru", meaning "to be able to." The polite present form of dekiru is dekimasu.

To express being able to do something, add koto ga dekimasu to the plain form of a verb. After a noun, just add "ga dekimasu."

マイクさんは漢字を書くことができますか。　（マイクさんは漢字を書けますか）
Maiku san wa kanji o kaku koto ga dekimasu ka. (Maiku san wa kanji o kakemasu ka.)
Mike, can you write kanji characters?

日本語を話すことができますか。 (日本語が話せますか。)
Nihongo o hanasu koto ga dekimasu ka. (Nihongo ga hanasemasu ka.)
Can you speak Japanese?

日本語を書くことができません。
Nihongo o kaku koto ga dekimasen.
I cannot write Japanese.

日本語ができません。
Nihongo ga dekimasen.
I do not know Japanese.

Potential form of a verb

The potential form of a verb indicates the ability to do it or the possibility of doing it. Let's look at some examples.

日本語を話せますか？
nihongo o hanasemasuka?
Can (you) speak Japanese?

人間は時速50きろで走れますか？
Ningen wa jizoku 50 kiro de hashiremasuka?
Can a human run 50 km/h?

For ichidan (ru-dropping) verbs, the potential form is made by replacing the last syllable -ru by -rareru. (This is the same as the passive form for these verbs). For example, taberu -> taberareru, which can mean either "can eat" (potential) or "can be eaten" (passive).

For godan (u-dropping) verbs, use the stem ending in the vowel sound "e" and add -ru. In the examples above, hanasu -> hanaseru, hashiru -> hashireru.

Once you have the potential form obtained above, it inflects like an ichidan verb; so the polite present and past forms of hanaseru are hanasemasu, hanasemasen, hanasemashita, hanasemasen deshita, and its -te form is hanasete.

The potential form of kuru is kureru; for suru, you will usually use dekiru (explained above) as if it were the potential form of suru.