The other night I saw an old friend here in Tokyo, one of my coworkers from when I was teaching English.

She was telling me a story about how/why she broke up with her boyfriend, and in that story this sentence popped up:

終電逃した。
しゅうでん　のがした。
I missed my last train. // I stayed out past my last train.
Literally: "last train + *set free / let escape."

Maybe I'm just a language nerd, but I thought that the use of the verb 逃す（のがす） was so cool here.

Or at least, I used to think it was cool until I realized that Japanese dictionaries had betrayed my trust once again. They're like that unreliable lover you keep giving second chances to--always letting you down.

Take these two dictionary entries, for instance:

逃す（のがす // to miss; to lose; to give away; to let go; to set free; to let get away）
逃がす（にがす // to miss; to lose; to give away; to let go; to set free; to let get away）

So are のがす and にがす the same, or what? Or do we only use one of them in commonly spoken Japanese?

No and no.

Although Japanese dictionaries do not differentiate these words, you'll find that in everyday, spoken Japanese, these two words are used VERY differently. Here are my revised definitions:

逃す（のがす // to miss; to lose; to give away）
逃がす（にがす // to let go; to set free; to let get away）

Sadly, with my new definitions, we can no longer say that we "set the last train free." Instead we just "missed the last train" or "lost (our chance for) the last train." How boring.

Conversely, you cannot 逃がす（にがす） the last train. But if you catch a tiny, scared animal in the forest, you could "set it free," which would be to 逃がす（にがす）. Here are some examples...

終電逃しちゃった。
しゅうでん　のがしちゃった。
I missed my last train.
Literally: "last train + missed."
Note: This is almost the same sentence as earlier, but by saying 逃しちゃった, which is the casual form of 逃してしまいました or 逃してしまった, then there is the added nuance that this was a mistake (or maybe not a very good thing). In the first sentence, missing the train may have been intentional, which is why I also included the translation "I stayed out past my last train."

昇進するチャンスを逃してしまった。
しょうしんする　チャンス　を　のがして　しまった。
I missed my chance to get a promotion.
Literally: "promotion + do + chance + を + missed."

Now compare those two examples to the following two examples, which use 逃がす（にがす）.

まだ子供だから逃がしてあげよう。
まだ　こども　だ　から　にがして　あげよう。
It's still a kid, so let's let it go. // Let's set it free since it's still a kid.
Literally: "still + child + is + because + set free + let's give."
Note: For example, if you caught a little fish or a tiny rabbit or something, you might set it free since it's still small.

逃がさないように見張ってろ。
にがさない　ように　みはってろ。
Watch him so he doesn't escape. // Don't let him escape.
Literally: "does not escape + so that + watch him."
Note: Using the plain form a verb with ～ように is like saying "so that VERB." In this case, it's "does not escape + ように," which means it's "so that (he) doesn't escape." Also, 見張ってろ is a shortened form of 見張っていろ, which is a combo of the verb 見張って, "watch over" and the imperative form of the auxiliary verb いる, which in this case just means "keep doing" or "stay doing." So the the nuance is (1) "keep watching over the prisoner" and (2) "this is an order."

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Unless you're a wild game hunter, chances are you'll be using 逃す（のがす） much more often than you use 逃がす（にがす）.

Interestingly, 逃す（のがす） can also attach to other verbs, in which case it takes on the meaning of "to miss/lose the chance to VERB." Here are the two most common examples:

今回は見逃してあげる。
こんかい　は　みのがしてあげる。
I'll overlook it this time.
Literally: "this time + は + overlook (= see + miss) + give."
Note: For example, a boss could tell an employee that he'll "miss seeing" a mistake.

大事な部分を聞き逃した。
だいじな　ぶぶん　を　ききのがした。
I missed an important part. // I didn't catch an important part.
Literally: "important + part + を + missed hearing."
Note: For example, you could say this if you missed an important part of a Japanese listening test/exercise.

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逃す（のがす） and 逃がす（にがす） are both transitive verbs, but most people will learn the intransitive version of these first.

Do you know what it is?

逃げる（にげる // to run away; to escape; to flee）

Here are some examples:

警察だ！逃げろ！
けいさつ　だ！　にげろ！
It's the cops! Run!
Literally: "police + is! + run away!"

今のうちに逃げよう。
いま　の　うちに　にげよう。
Now's our chance to escape.
Literally: "now + の + うちに (= in this span of time) + let's escape/run away."

うちに is used to express a finite span of time. Whatever comes before うちに is the condition or situation that will not last for very long. For example:

あったかいうちに食べてね。
あったかい　うちに　たべて　ね。
Eat it while it's warm.
Literally: "warm + うちに + eat + ね."
Note: あったかい is a casual abbreviation of the word 温かい（あたたかい //warm）.

On a side note, there are two kanji for あたたかい (="warm"): 温かい and 暖かい. Use 温かい for warm things you can touch (like food and water) and use 暖かい for warm things you can't touch (like the weather and the air in a room).

ライオンが動物園から逃げたって。
ライオン　が　どうぶつえん　から　にげた　って。
I heard a lion escaped from the zoo. // They're saying a lion escaped from the zoo.
Literally: "lion + が + zoo + from + escaped + って."

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Anyways, if you come to Tokyo, be sure that you don't 逃す（のがす） your last train... unless you want to stay out until the trains start again at 5 or 6 in the morning... or pay for an obscenely expensive taxi ride home.