What Is the Difference Between 「まい」 and 「ないだろう」? How to Handle Older Negative Conjecture in Japanese Reading

When reading Japanese, you may sometimes come across the expression 「まい」.

For example:

「彼はもう戻るまい。」
“He probably will not come back anymore.”

This expression is not very common in everyday modern conversation.
Because of that, many Japanese learners stop and wonder, “What does this mean?”

The good news is that 「まい」 is not as difficult as it first looks.

In many cases, 「まい」 can be understood as an older or more formal version of 「ないだろう」.

So:

「彼はもう戻るまい。」
can be understood as:
「彼はもう戻らないだろう。」
“He probably will not come back anymore.”

Once you learn this basic conversion, 「まい」 becomes much easier to read.

「ないだろう」 is a modern and natural way to express negative conjecture

Let’s start with 「ないだろう」.

「ないだろう」 is a common modern Japanese expression used to make a negative guess or prediction.

It means something like:

“probably not”
“I don’t think it will”
“it is unlikely that…”

For example:

「明日は雨が降らないだろう。」
“It probably will not rain tomorrow.”

「彼はその話を知らないだろう。」
“He probably does not know about that.”

「この問題は試験には出ないだろう。」
“This question probably will not appear on the exam.”

In all of these examples, the speaker is making a judgment based on the situation and predicting that something will not happen or is not true.

So 「ないだろう」 is a normal, modern expression for negative conjecture.

「まい」 is older, more formal, and more literary

Now let’s look at 「まい」.

「まい」 sounds older, more formal, and sometimes literary.
You may see it in written Japanese, formal essays, older texts, novels, or JLPT reading passages.

In one of its main uses, 「まい」 means:

「〜ないだろう」
“probably will not…”

For example:

「この寒さでは、桜はまだ咲くまい。」
“With this cold weather, the cherry blossoms probably will not bloom yet.”

This can be understood as:

「この寒さでは、桜はまだ咲かないだろう。」
“With this cold weather, the cherry blossoms probably will not bloom yet.”

Another example:

「彼ほどの人が、そんな初歩的なミスをするまい。」
“A person like him probably would not make such a basic mistake.”

This is close to:

「彼ほどの人が、そんな初歩的なミスをしないだろう。」
“A person like him probably would not make such a basic mistake.”

And:

「今から出ても、もう間に合うまい。」
“Even if we leave now, we probably will not make it in time.”

This can be read as:

「今から出ても、もう間に合わないだろう。」
“Even if we leave now, we probably will not make it in time.”

When you see 「まい」 in a reading passage, first try replacing it with 「ないだろう」.
In many cases, the meaning will become clear immediately.

But 「まい」 can also express negative intention

However, 「まい」 does not always mean 「ないだろう」.

It can also express the speaker’s negative intention.

In this use, 「まい」 means:

「〜しないつもりだ」
“I will not…”
“I have decided not to…”

For example:

「二度と同じ失敗はするまい。」
“I will not make the same mistake again.”

This means:

「二度と同じ失敗はしないつもりだ。」
“I intend not to make the same mistake again.”

Another example:

「もう彼には頼るまい。」
“I will not rely on him anymore.”

This means:

「もう彼には頼らないつもりだ。」
“I have decided not to rely on him anymore.”

And:

「このことは誰にも話すまい。」
“I will not tell anyone about this.”

This means:

「このことは誰にも話さないつもりだ。」
“I intend not to tell anyone about this.”

In these examples, 「まい」 is not a prediction.
It expresses the speaker’s decision or determination.

This is an important difference from 「ないだろう」.

The key question: prediction or determination?

When you see 「まい」, ask yourself this:

Is the sentence making a prediction?
Or is the speaker expressing determination?

If the sentence is about someone else or an outside situation, 「まい」 often means 「ないだろう」.

For example:

「彼は来るまい。」
“He probably will not come.”

This is close to:

「彼は来ないだろう。」
“He probably will not come.”

Here, the speaker is making a prediction about another person.

But if the sentence is about the speaker’s own action, 「まい」 may mean 「しないつもりだ」.

For example:

「もう迷うまい。」
“I will not hesitate anymore.”

This is close to:

「もう迷わないつもりだ。」
“I have decided not to hesitate anymore.”

So the basic distinction is this:

If it is a prediction about what will not happen, read 「まい」 as 「ないだろう」.
If it is a strong decision by the speaker, read 「まい」 as 「しないつもりだ」.

「ないだろう」 is not old. 「まい」 sounds written and formal

「ないだろう」 is natural in modern Japanese.

You can use it in conversation, explanations, comments, and writing.

For example:

「彼は今日は来ないだろう。」
“He probably will not come today.”

「この方法ではうまくいかないだろう。」
“This method probably will not work.”

On the other hand, 「まい」 sounds more formal or literary.

「彼は今日は来るまい。」
“He probably will not come today.”

「この方法ではうまくいくまい。」
“This method probably will not work.”

These sentences are understandable, but they sound more written, old-fashioned, or literary than ordinary conversation.

So when you speak or write in modern Japanese, you can usually use 「ないだろう」.
For 「まい」, it is enough to understand it when it appears in reading.

How to handle 「まい」 in reading passages

When 「まい」 appears in a reading passage, use this simple method.

First, find the verb before 「まい」.

Then, try replacing 「まい」 with 「ないだろう」.

For example:

「成功するまい」
means:
「成功しないだろう」
“It probably will not succeed.”

「許されまい」
means:
「許されないだろう」
“It probably will not be forgiven.”

If this reading makes sense, then 「まい」 is being used as negative conjecture.

But if the subject is the speaker, and the sentence feels like a strong decision, try reading it as 「しないつもりだ」.

For example:

「忘れるまい」
means:
「忘れないつもりだ」
“I will not forget.”

「負けるまい」
means:
「負けないつもりだ」
“I will not lose.”

This two-step method is very useful.

First, try 「ないだろう」.
If that does not fit, try 「しないつもりだ」.

「あるまい」 is a common form

One form that often appears is 「あるまい」.

For example:

「そんなことはあるまい。」
“That probably is not true.”

This can be understood as:

「そんなことはないだろう。」
“That probably is not true.”

Another example:

「問題はあるまい。」
“There probably will be no problem.”

This is close to:

「問題はないだろう。」
“There probably will be no problem.”

「あるまい」 is a useful expression to recognize because it appears relatively often in formal writing and reading passages.

Summary

「まい」 and 「ないだろう」 can both express a negative prediction.

However, they are not exactly the same.

「ないだろう」 is modern, natural, and commonly used.
It means “probably not” or “will probably not.”

「まい」 is older, more formal, and more written.
In many cases, it means 「ないだろう」, but it can also express the speaker’s determination not to do something.

So when you see 「まい」, remember these two possibilities.

「〜まい」 can mean:
「〜ないだろう」
“probably will not…”

Or it can mean:
「〜しないつもりだ」
“I will not…”

The best strategy is simple.

First, replace 「まい」 with 「ないだろう」.
If that does not fit, try reading it as 「しないつもりだ」.

With this method, even older or more formal expressions become much easier to understand.

RJT helps Japanese learners understand confusing grammar patterns like 「まい」 and 「ないだろう」 through clear explanations, examples, and practice questions.
If you want to build stronger reading skills and understand tricky JLPT grammar more naturally, continue learning with RJT.

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