Lesson 12: Expressing Can & Must (Potential／Obligation)

Expressing Ability with ことができる

This is a formal and structured way to express "can do something" in Japanese.

How to Form

Verb (dictionary form) + ことができる
Polite form: Verb + ことができます

Negative form: Verb + ことができない (casual) / ことができません (polite)

Examples:

Usage Notes

• ことができる is more formal and is common in business, academic, or official contexts
• The structure literally means "to be able to do the act of [verb]"
• Works well with noun + する combinations:
  サッカーをすることができます (I can play soccer)
• For negative ability, simply negate できる:
  話はなすことができません (I cannot speak)

Cultural Note:
This formal pattern is frequently used in job interviews when discussing skills
「何なにができますか？」
"What can you do?"
It's also common in official signs and announcements
「ここで切符きっぷを買かうことができます」
"You can buy tickets here."

Expressing Ability with Potential Form

The potential form is a more natural, everyday way to express ability in Japanese.

How to Form by Verb Group

Group 1 (う-verbs)

Change final う-sound to corresponding え-sound + る

Group 2 (る-verbs)

Drop る and add られる

Note: In casual speech, many speakers drop the ら, saying
食たべれる
(tabereru)
or
見みれる
(mireru)

Group 3 (Irregular Verbs)

• する → できる
• 来くる → 来こられる

Negative Potential Form

Simply add ない to the potential form:

• 話はなせる → 話はなせない ("cannot speak")
• 食たべられる → 食たべられない ("cannot eat")
• できる → できない ("cannot do")

For polite speech, use ません:

• 話はなせません
• 食たべられません
• できません

Examples:

Everyday Usage:
The potential form is extremely common in daily conversation:
「行いける？」
"Can you go?"
「見みられる？」
"Can you see it?"
「それ、食たべられる？」
"Can you eat that?"

Particle Shift with Potential Form (を → が)

When using the potential form, the direct object marker typically changes from を to が.

Compare Normal vs. Potential

Normal Form (を)

Potential Form (が)

Why the Change?

The particle shifts because the object is now viewed as the subject of ability.
In the potential form, the focus changes from "I do X" to "X is doable by me."

Verbs That Don't Take Direct Objects

Note that verbs that don't normally take an object with を don't need this particle change:

Usage Note:
While が is considered standard with potential forms in textbooks and formal Japanese,
you might hear some native speakers use を in casual conversation.
For example: 日本語にほんごを話はなせる
Both are understood, but が is generally taught as the correct form.

Expressing Ability with Special Verbs

Some common verbs have special ways of expressing ability or use different patterns.

Noun + することができる/できる

For activities expressed as noun + する verbs, we can directly use できる:

わかる vs. 分かれる

The verb わかる (to understand) already contains the meaning of ability, so it doesn't use the potential form:

できる as a Standalone Verb

できる can also function as a standalone verb meaning "can do" or "to be able to":

Cultural Note:
When Japanese people are asked if they can do something, they often respond modestly
even if they're quite skilled. Phrases like
まあまあできます ("I can do it somewhat well") or
少しだけ ("just a little bit")
are common responses that show humility.

Expressing Obligation with なければならない

This is a formal way to express "must" or "have to" in Japanese.

How to Form

1. Start with the negative plain form of the verb (〜ない)
2. Add なければならない (casual) or なければなりません (polite)

Step-by-Step Formation

食たべます
(tabemasu)
↓
食たべない
(tabenai)
↓
食たべなければならない
(tabenakereba naranai)

行いきます
(ikimasu)
↓
行いかない
(ikanai)
↓
行いかなければならない
(ikanakereba naranai)

Examples:

Negative Form (Don't Need To)

To express "don't need to" or "don't have to," use なくてもいい:

Cultural Insight:
なければならない is somewhat formal and official-sounding.
It's common in:
Written instructions or manuals
Official rules and regulations
Formal business contexts
Expressing strong necessity or obligation

The expression literally means "if (I) don't do X, it won't become/work."

Expressing Obligation with ないといけない

A more conversational way to express "must" or "have to" in Japanese.

How to Form

1. Start with the negative plain form of the verb (〜ない)
2. Add といけない (casual) or といけません (polite)

Examples:

Meaning Breakdown

This expression literally means "if [I] don't do X, it's not good/won't work."

• 行いかない = don't go
• と = if
• いけない = no good/won't work

Variations

There are several similar patterns with the same meaning:

• ないといけない (most common)
• ないとだめ(だ/です) (slightly more casual)
• ないとならない (similar nuance to なければならない)

Usage Note:
ないといけない is more conversational and natural in everyday speech than なければならない.
It's less stiff while still being appropriate for most situations.

Putting It All Together: Practical Applications

Let's see how these ability and obligation expressions are used in different contexts.

Expressing Ability in Daily Life

Expressing Obligation in Various Situations

Dialogue: Job Interview

A: どんな言語げんごが話はなせますか？
(Donna gengo ga hanasemasu ka?)
"What languages can you speak?"

B: 日本語にほんごと英語えいごが 話はなせます。
中国語ちゅうごくごも少すこし話はなすことができます。
(Nihongo to eigo ga hanasemasu. Chūgokugo mo sukoshi hanasu koto ga dekimasu.)
"I can speak Japanese and English. I can also speak a little Chinese."

A: パソコンは使つかえますか？
(Pasokon wa tsukaemasu ka?)
"Can you use a computer?"

B: はい、Excelと日本語にほんごのワープロが使つかえます。
(Hai, Excel to nihongo no wāpuro ga tsukaemasu.)
"Yes, I can use Excel and Japanese word processing."

A: いつから働はたらけますか？
(Itsu kara hatarakemasu ka?)
"When can you start working?"

B: 来週らいしゅうから働はたらくことができます。
(Raishū kara hataraku koto ga dekimasu.)
"I can start working from next week."

Dialogue: Making Plans

A: 明日あした、映画えいがを見みに行いきませんか？
(Ashita, eiga o mi ni ikimasen ka?)
"Would you like to go see a movie tomorrow?"

B: すみません、明日あしたは行いけません。
宿題しゅくだいをしなければなりません。
(Sumimasen, ashita wa ikemasen. Shukudai o shinakereba narimasen.)
"I'm sorry, I can't go tomorrow. I have to do my homework."

A: そうですか。では、週末しゅうまつはどうですか？
(Sō desu ka. Dewa, shūmatsu wa dō desu ka?)
"I see. Then, how about the weekend?"

B: 週末しゅうまつなら行いけると思おもいます。
土曜日どようびは何時なんじに会あいますか？
(Shūmatsu nara ikeru to omoimasu. Doyōbi wa nanji ni aimasu ka?)
"I think I can go on the weekend. What time shall we meet on Saturday?"

A: 2時じに駅えきで会あいましょう。
2時じに来こられますか？
(Ni-ji ni eki de aimashō. Ni-ji ni koraremasu ka?)
"Let's meet at the station at 2 o'clock. Can you come at 2?"

B: はい、2時じに行いけます。
楽たのしみにしています。
(Hai, ni-ji ni ikemasu. Tanoshimi ni shite imasu.)
"Yes, I can go at 2. I'm looking forward to it."

Dialogue: Explaining Requirements

A: この書類しょるいはいつまでに提出ていしゅつしなければなりませんか？
(Kono shorui wa itsu made ni teishutsu shinakereba narimasen ka?)
"By when must I submit this document?"

B: 金曜日きんようびまでに提出ていしゅつしなければなりません。
遅おそれないでください。
(Kinyōbi made ni teishutsu shinakereba narimasen. Okurenaide kudasai.)
"You must submit it by Friday. Please don't be late."

A: 金曜日きんようびは休やすみですが、
木曜日もくようびに提出ていしゅつできます。
(Kinyōbi wa yasumi desu ga, mokuyōbi ni teishutsu dekimasu.)
"Friday is a day off, but I can submit it on Thursday."

B: はい、木曜日もくようびでもいいです。
忘わすれないでくださいね。
(Hai, mokuyōbi demo ii desu. Wasurenaide kudasai ne.)
"Yes, Thursday is also fine. Please don't forget."

Cultural Note:
In Japanese culture, expressing inability to do something often comes with an explanation of obligation.
Rather than simply saying "I can't," it's polite to explain why by mentioning what you must do instead.
Similarly, when discussing requirements, giving specific timeframes is preferred to vague deadlines.