As it’s one of the first words most students learn, it can be surprising to discover just how tricky all the uses of こと in Japanese actually are. While it’s in most beginner’s textbooks, this critical particle will also be waiting for them when they reach the intermediate- to advanced-level Japanese, too! As learners looking to move from JLPT3 to JLPT2 grapple with the umpteenth meaning of this word, it can seem that learning to master こと is like learning to master the 箏（こと）（or 琴) (Japanese harp)!
For beginners, one of the first grammar points that must be mastered is the use of こと to make a noun phrase. For those like me, who skipped linguistics classes whenever possible, a noun phrase is defined as a word or group of words containing a noun and functioning in a sentence as subject, object or prepositional object.
A reasonably simple example of this use is in the sentence: “私（わたし）は人前（ひとまえ）で話（はな）すことに慣（な）れた。” (“I got accustomed to speaking in public.”). Here, 人前で話すこと (literally: in front of people, speak, thing) is acting as the noun phrase. Without the こと, this would soon turn into a word salad with poorly defined verbs as a particularly unpleasant dressing.
This use of こと as a noun phrase is most often used with abstract verbs describing such things as internal thoughts and the transmission of information. As a result, learners will often encounter it in sentences using verbs such as 話（はな）す (to speak), 伝（つた）える (to convey) and 希望（きぼう）する (to wish). This is important to remember as it is one of the key differences that separate こと from の, which also forms noun phrases and often behaves in a similar way to こと.
Once you’re used to using the basic form of こと as a noun phrase, the next use that learners should master is the こただ-ending. This ending is often used for the purposes of emphasis. It is often found with sentences that offer advice or give suggestions for how to do something. A common example is ”無理（むり）をしないことだ。” (“Don’t push yourself.”).
Changing the end particle can change the meaning dramatically.
After that, learners will discover that changing the end particle can change the meaning dramatically. Changing the particle after こと to に, for example, creates the meaning of an emotional reaction to something. This is found with past tense verbs, for example, 驚（おどろ）いたことに (surprisingly) and 残念（ざんねん）なことに (unfortunately).
Another common variant is the ことか-ending. The か part of this ending is often found with questions, so unsurprisingly the most common use of ことか is to make a kind of rhetorical question such as in the sentence: ”何度注意（なんどちゅうい）したことか？” (“How many times have I warned you?”).
One of the big surprises for me was that the two other common forms ことがない and ことはない that look very similar, actually have surprisingly different meanings. ことがない basically means that something never occurs. For example,” 嘘（うそ）を言（い）ったことがない” means “I have never told any lies” but conversely, ことはない means “no need to~”. So, 何（なに）も心配（しんぱい）することはない is “Nothing to worry about.”
こと is a fascinating word as by slightly changing the particle, the meaning changes dramatically. Unfortunately for learners making the transition from intermediate to advanced Japanese, all of these endings need to be mastered.
Conveniently, there is yet another use of こと that perfectly sums up this taxing problem for learners: “失敗（しっぱい）を繰（く）り返（かえ）すことで、成功（せいこう）に至（いた）る.” (“By repeating failure, it leads to success.”). Something to bear in mind as you try to master all of these tricky uses of こと!