You may have come across potential form (or potential verb) in Japanese if you are at the intermediate level. It's one of the 2 ways to express ability and possibility to do something in Japanese. Another way is to convert a verb into its noun-form and append ができる (ga dekiru) to it.
It's very simple to convert a Japanese verb into its noun-form. To do that, simply append こと(koto) to the dictionary-form of any verb. For example, 話す (hanasu) becomes 話すこと (hanasu koto), 読む (yomu) becomes 読むこと (yomu koto).
By appending ができる (ga dekiru) to a noun or a verb in its noun-form, as shown in the following sentence patterns, you can express the ability and possibility of performing an action.
A. Noun ができる (Noun ga dekiru)
B. Verb + こと ができる (Verb+koto ga dekiru)
Let's use some examples to show how to use the above sentence patterns...
watashi wa tenisu ga dekimasu
Meaning: I can play tennis. (ability to play tennis)
watashi wa suki- ga dekimasu
Meaning: I can ski. (ability to ski)
watashi wa nihongo wo hanasu koto ga dekimasu
Meaning: I can speak Japanese. (ability to speak Japanese)
koko de shashin wo torimasu
Meaning: I take photo here. (action to take photo)
koko de shashin wo toru koto ga dekimasu
Meaning: I can take photo here. (possibility to take photo)
To express ability or possibility, we have one more expression... Potential verb or Potential form. The meaning of "can" has already been included in the verb without using できる.
The following are the rules for the 3 groups of Japanese verbs.
Group 1 verbs always end up with words containing u-sound. To change group 1 verbs to potential-form, change the u-sound to e-sound in the last word. After that append る (ru) to the changed words and you will get the potential-form of the verb.
For example, this group 1 verb よむ (yomu) ends up with む (mu). If you change the u-sound to e-sound, む (mu) becomes め (me). Adding る (ru) at the end, you will get よめる (yomeru).
Let's look at one more example, this group 1 verb はしる (hashiru) ends up with る (ru). If you change the u-sound to e-sound, る (ru) becomes れ (re). Adding る (ru) at the end, you will get はしれる (hashireru).
Changing group 2 verbs from dictionary-form to potential-form is much simpler. All group 2 verbs end up with る (ru). To change to potential-form, simply replace る (ru) with られる (rareru) and you will get the potential-form of the verb.
For example, this Japanese verb たべる (taberu) is a group 2 verb. To change to potential-form, replace る (ru) with られる (rareru) and you will get たべられる (taberareru).
I will use another example to show the change. This group 2 verb ねる (neru) means go to bed. Change る (ru) to られる (rareru) and you will get ねられる (nerareru).
Group 3 verbs are irregular verbs. You just have to memorize them. The potential-form of くる (kuru) is こられる (korareru) and the potential-form of する (suru) is できる (dekiru).
In summary, the following diagram shows the rules when changing dictionary-form to potential-form for Japanese verbs.
So you have 2 ways to express the meaning of ability or possibility in Japanese. For example, the following 2 sentences mean the same thing... "I can speak Japanese".
Note: When using potential form, you have to change the particle from を (wo) to が (ga). This was what I were taught by my teacher in the beginning. However, I later found out that this is not always the case. Depending on situations, を (wo) can also be used. We were told to use が (ga) because our teacher didn't want us to be confused when we were first learning the potential form. So in this lesson, I will still use が (ga) for the particle.
And since all potential verbs are Group 2 verbs, it's easy to change a potential verb to its other forms. Following the rules for group 2 verbs,
For example, using the potential verb 話せる (hanaseru), which means "can speak"...
Potential verbs can also be applied to the polite style. Using the same example,
Let's practise using both ways to express ability or possibility with more examples...
ani wa oyogu koto ga dekimasu
ani wa oyogemasu
Meaning: My elder brother can swim.
haha wa nihon ryouri wo tsukuru koto ga dekimasu
haha wa nihon ryouri ga tsukuremasu
Meaning: My mother can cook Japanese dish.
otouto wa jitensha ni noru koto ga dekimasu
otouto wa jitensha ni noremasu
Meaning: My younger brother can ride the bicycle.
ashita pa-ti- ni iku koto ga dekimasen
ashita pa-ti- ni ikemasen
Meaning: I can't go to the party tommorrow.
kinou bangohan wo taberu koto ga dekimasendeshita
kinou bangohan ga taberaremasendeshita
Meaning: I couldn't eat dinner yesterday.