あく, あける, and ひらく all mean “to open,” but they’re used in different contexts.
To understand these verbs, it’s important to disentangle the questions of what the verbs themselves mean and how they are written in kanji. Even if there were no such thing as kanji, these would still be three different verbs with different usages.
So let’s first consider them in hiragana only. Then we’ll come back and look at how to tell them apart when they are written in kanji.
Differences In Usage
Of these three verbs, two are related. They belong to a transitive-intransitive pair (similar to the pairs とめる・とまる and しめる・しまる).
あける to open (transitive)
あく to open (intransitive)
That is to say, you use them as follows:
ジョンさんはドアをあける。
John opens the door. (transitive)
ドアがあく。
The door opens. (intransitive)
The verb ひらく is the odd man out. It’s also special, and somewhat rare among Japanese verbs, in that it can function both transitively and intransitively. So you can say, for example:
ジョンさんはドアをひらく。
John opens the door. (transitive)
ドアがひらく。
The door opens. (intransitive)
But if ひらく can functon both transitively and intransitively, you might wonder, it seems like it can do the job of both あける and あく. That sounds more convenient, right? Why not just use ひらく exclusively and call it a day? In many cases, you can indeed do just that. However, ひらく feels slightly different from the あく・あける pair.
Difference In Meaning
ひらく feels more like a three-dimensional opening up, an unfolding out into space, of something that is made to be opened and closed, like a door, an umbrella, a book, or a flower.
In contrast, the あく・あける pair feels simpler and more two-dimensional, like a panel that slides open. Additionally, it is used for things that open one time but can’t be closed again (like a can of corn).
Ultimately, it comes down to the specific thing being opened. Some things open with あく・あける, and some open with ひらく. But several of the most common everyday objects — doors, windows, eyes — can be opened with either. And in those cases, you can use whichever you like better.
Telling Them Apart In Print
Now that we know how to use these verbs, let’s consider how to tell them apart when they are written in kanji. These three verbs are all written using the same character (開). So, if we write each verb in kanji, they look like this:
開ける（あける）
開く （あく）
開く （ひらく）
As you can see, if you see an okurigana け, then you can know for sure that the verb is あける. It cannot be anything else. It’s あける. Case closed.
But あく and ひらく are written in exactly the same way, and that’s true for their other forms like the past tense and negative (開いた and 開かない, respectively). So, when you encounter one of those in print, how can you tell which verb is meant?
First, there are hints from the structure of the sentence. So if the sentence is ドアを開く, it can only be ひらく, because the verb is functioning transitively. あく is ruled out because it is intransitive.
However, when you see a sentence like ドアが開く, where the verb is functioning intransitively, あく and ひらく could both grammatically fit there, so neither is ruled out.
If it were 傘（かさ）が開く, “the umbrella opens,” we would know for sure that it must be ひらく, because that is the appropriate verb for umbrellas. (Recall that ひらく feels more like a three-dimensional opening up in space.)
But in a case like ドアが開く, where both verbs fit grammatically and both are appropriate to the item in question, then you are free as the reader to choose whichever verb you like. You can read it ドアがあく or ドアがひらく. Both are equally correct.
Conclusion
Hopefully this article helped you understand the differences between あく, あける, and ひらく.
Seeing these words in context will help to further cement their meanings into your brain, so reading a lot in Japanese is crucial to mastering vocabulary like this.
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