In short

Japanese says “can” by changing the verb itself: 話す hanasu (“speak”) -> 話せる hanaseru (“can speak”). Two things move at once - the verb takes the potential ending, and the object particle を normally flips to が: 日本語が話せます Nihongo ga hanasemasu (“I can speak Japanese”). The formal written equivalent is V-plain + ことができます.

The rule

Step 1 - Build the potential form from the verb group. You need the groups from Verb groups and the polite ます-form.

• Group I (五段): move the final kana from the -u row to the -e row and add る (く -> け, す -> せ, む -> め, つ -> て, う -> え). 書く kaku -> 書ける kakeru, 話す hanasu -> 話せる hanaseru, 読む yomu -> 読める yomeru, 待つ matsu -> 待てる materu.
• Group II (一段): drop る and add られる. 食べる taberu -> 食べられる taberareru, 見る miru -> 見られる mirareru.
• する -> できる dekiru. Completely suppletive - there is no ×される in this meaning.
• 来る -> 来られる korareru. The reading stays ko-, exactly as in 来ない konai.

Step 2 - Every potential verb is a Group II verb. Whatever it was before, once it ends in -eru or -られる it conjugates like 食べる: 話せます, 話せません, 話せた, 話せて, 話せない. This is a free gift - one paradigm covers all potentials, including できる.

Step 3 - The object particle shifts: を -> が. With a potential verb, what was the direct object usually becomes the subject of a state, so it takes が.

• 日本語を話します Nihongo o hanashimasu (“I speak Japanese”) -> 日本語が話せます Nihongo ga hanasemasu (“I can speak Japanese”).

が is the safe default while you are learning, but を does survive in three situations:

1. Inside ことができる - there the inner verb is an ordinary transitive verb, so it keeps を: 日本語を話すことができます.
2. When the subject is already marked が, to avoid two が in one clause: 私が漢字を読めます.
3. When the object phrase is long or separated from the verb by other material.

Step 4 - Potential verbs are stative, so treat them like adjectives of ability. They describe a state, not an event. Do not put 〜ている on them for a plain ability: ×英語が話せています -> 英語が話せます. (You will later meet 〜ています on potentials meaning “has managed to so far”, as in まだ読めていません - leave it alone for now.) For the same reason, some already stative verbs have no potential of their own: 分かる wakaru and ある aru stay as they are - say 分かります, not ×分かれます (分かれる is a different verb, “to split, to branch”). いる is the exception worth knowing: いられる irareru does exist, but it means “can stay, can remain” (ここにいられますか), not “can exist”.

Step 5 - Spontaneous 見える／聞こえる versus potential 見られる／聞ける. Japanese splits “can see” and “can hear” in two, and the difference is whether anyone chose it.

• 見える mieru / 聞こえる kikoeru - it comes into view or reaches your ears by itself. No volition, no arrangement: the mountain is simply there, the music is simply audible.
• 見られる mirareru / 聞ける kikeru - circumstances allow you to watch or listen: the museum is open, the concert has tickets, the match is on tonight.

So 部屋から海が見えます is about your eyes and the window; 今夜テレビで試合が見られます is about the schedule. Both take が - 見える and 聞こえる are intransitive and never take を at all.

Step 6 - The formal equivalent: V-plain + ことができます. Nominalize the clause with こと (see Nominalization with こと and の) and finish with できます:

ここで写真を撮ることができます。 Koko de shashin o toru koto ga dekimasu.

Note the register split, the point that catches everyone: the verb before こと is plain even though the sentence ends politely. ×撮りますことができます is wrong. This pattern is longer, heavier and more formal - it belongs to notices, rules, manuals and written explanations. In conversation the plain potential verb is the normal choice.

Step 7 - する-nouns take できる directly. Because する -> できる, any noun that runs on する gets its potential for free: 運転する -> 運転できます unten dekimasu, 予約する -> 予約できます. You may also insert が: 運転ができます. But do not use を here: ×運転をできます -> 運転ができます, or the full 運転をすることができます.

Step 8 - Negative potential is the polite way to say no. Japanese declines by reporting inability, not unwillingness. 行きません ikimasen (“I won’t go”) sounds willful; 行けません ikemasen (“I can’t go”) is the courteous form, and ちょっと chotto softens it further: すみません、明日はちょっと行けません. The bare ちょっとできません chotto dekimasen is a standard, face-saving refusal - it means “no” without ever saying it.

A form you will meet again. For Group II verbs and 来る, the potential 食べられる／来られる is spelled and pronounced exactly like the passive (and the honorific). Only the particles and the context tell them apart; that disambiguation is taught in Passive 〜（ら）れる. Group I verbs have no such clash: 話せる (potential) and 話される (passive) are visibly different.

Step 9 - ら抜き言葉. In everyday spoken Japanese, Group II and 来る potentials often drop the ら: 食べられる -> 食べれる tabereru, 見られる -> 見れる mireru, 来られる -> 来れる koreru. This is extremely widespread and you will hear it constantly - it also conveniently removes the clash with the passive. It is still avoided in writing, in exams and in formal speech. Understand it; produce the full form.

Forms & patterns

Deriving the potential:

Conjugating a potential verb - always Group II:

Three ways to say “can”:

Spontaneous versus potential:

Examples

Common mistakes

• 日本語を話せます。 (Nihongo o hanasemasu.) -> 日本語が話せます。 (Nihongo ga hanasemasu.) - the potential turns the object into the subject of a state, so を becomes が. Keep を only inside ことができる or to avoid a second が.
• ピアノを弾けることができます。 (Piano o hikeru koto ga dekimasu.) -> ピアノを弾くことができます。 (Piano o hiku koto ga dekimasu.) - ことができる already means “can”, so the verb in front of こと is the plain dictionary form. Never stack two potentials.
• きられます。 (Kiraremasu.) -> 来られます。 (Koraremasu.) - 来る is irregular and keeps the ko- reading throughout the potential, just as it does in 来ない konai.
• ホテルから海が見られます。 (Hoteru kara umi ga miraremasu.) -> ホテルから海が見えます。 (Hoteru kara umi ga miemasu.) - nobody arranged for the sea to be there; when something is simply within sight, use the spontaneous 見える. 見られる needs circumstances that permit watching.
• 英語が話せています。 (Eigo ga hanasete imasu.) -> 英語が話せます。 (Eigo ga hanasemasu.) - potential verbs are stative. An ability is expressed by the simple 〜ます form, not by 〜ている.

Tips

• One rule covers Group I: put the verb in the ば-shape, then add る. 書く -> 書け(ば) -> 書ける, 読む -> 読め(ば) -> 読める. If you know the -e row of the kana chart, you already know every Group I potential.
• できる is the answer to three different questions. As a potential it means “can do”; on its own it also means “be completed” (レポートができました, “the report is done”) and “come into being” (新しい駅ができます, “a new station is going up”). Context, not form, separates them.
• 行けません is not いけません. 行けません is “I can’t go” - the potential of 行く. いけません, written in kana, is the “it won’t do” of 〜てはいけません from Obligation, necessity and advice. They sound identical, so the kanji is doing real work.
• Refuse with “can’t”, never with “won’t”. 行きません and しません state a decision and sound cold. The polite move is the negative potential plus a hedge: ちょっと行けません, ちょっとできません. Learners who translate “I won’t” literally come across as far blunter than they intend.