First impressions matter — and sometimes they matter too much. Just look at one of the first things they teach us in Japanese: the ending ます. The verb part that comes before this ending is forever after referred to as "masu form", and nothing much is made of it other than occasionally mentioning that it can take additional endings such as たい or すぎる.
But did you know that this form of the verb is properly called the connective form (連用形, れんようけい), and has much a wider application than simply giving a ride to ます and a few other friends?
The primary function of the connective form is to enable verbs to be combined with other verbs to create a compound verb (複合動詞). The first verb in the compound appears in its connective form, and is the main element. Its basic meaning is extended by attaching the second verb, which appears in its full form. The connective form is invariable, and the compound verb is conjugated according to the second verb.
The importance of auxiliary verbs
There are two types of verbs that can directly follow the connective form: standard verbs and auxiliary verbs (補助動詞). Combinations with the first type are lexical; that is, they occur spontaneously as part of the natural growth of the vocabulary, just like any other word — so you can't make them up on your own. You have to use the ones that already exist in the dictionary.
On the other hand, combinations of a verb + auxiliary are grammatical, meaning that they can in principle be created freely by adding the auxiliary to any main verb that may logically take it. In practice, some auxiliaries are only combined with handful of verbs that have become strongly associated with them.
By the way, the te-form, which can take its own set of auxiliary verbs, was originally nothing more than a regular connective form followed by a て. This later transformed, by way of sound changes, into a distinct form that diverges in usage from the proper connective form. Auxiliaries that go with the te-form, such as いる and おく, are a much more basic part of the grammar, so I won't go into them here.
Auxiliary verbs add various functions and shades of meanings to the main verb. In fact, even the omnipresent ます is not an ending but an (irregular) auxiliary verb — one that adds the function of politeness and heightened social awareness. The same goes for other common elements that combine with the connective form, such as たい "to want to perform the main verb", すぎる "to perform the main verb excessively", or 始める "to begin performing the main verb"; all of these are auxiliaries and not merely endings.
The ability to correctly make verb combinations is an important part of being proficient in Japanese, and skillful use of auxiliaries is one of the things that determine how natural your speaking and writing will seem to natives.
Now that we know the basics of how the connective form functions, let's take a look at some of the more advanced auxiliary verbs. Knowing these will enable you to understand and use subtleties of Japanese that go way beyond the simpler meanings of the more common auxiliaries, so it's useful information to have regardless of your level.
Some in-depth examples
Used for actions that take place again and again, with a certain force that is maintained in each repeated action. Generally equivalent to adverbs such as "repeatedly", "incessantly", "vigorously", etc.
While usually written in hiragana, it can also appear in its kanji spelling — 頻る. The character itself means "frequent" and is included in the common word 頻度, "frequency".
This verb tends to be used in very specific contexts. Typical usage examples are 雨が降りしきる, "it is raining nonstop", and 鳴きしきる, "chirping incessantly" (said of birds).
Indicates the idea of missing the chance to do something. When combined with this auxiliary, the main verb represents an action that was not realized due to this missed opportunity.
Usage example: 言いそびれる, "missing the right moment to say something". This implies that the subject ultimately said nothing, because it was no longer possible or wise to speak up.
Also written 古す or 旧す, this verb indicates wearing something out by performing a certain action routinely over a long period of time. The idea is that the main verb's object is heavily affected by being used and is no longer in its original pristine state. This meaning can be literal, as in "worn out clothes", or figurative, as in "a hackneyed phrase".
Being connected with a description of some object has special implications for how this auxiliary verb is used: it typically appears in the た form and as part of an adjectival constructions (a phrase that precedes a noun).
Usage example: 着古したシャツ, "a well-worn shirt", "a shirt that has seen many years", etc.
Used for actions that are done "all over the place" — in a manner that is excessive, disorganized, unreasonable, etc. The idiomatic equivalent is "to go on and on" doing the main verb. Also written 捲る (the kanji means "to roll up").
Usage example: しゃべりまくる, "to talk on and on", "to ramble on".
This auxiliary verb indicates negative feelings such as contempt, hatred, displeasure, etc. toward the person who performs the main verb. Because of this unique function, やがる is only used when the speaker refers to other people, either in the second or the third person.
There's no equivalent for this verb in English, so it can only be translated indirectly by adding words from the general inventory of offensive vocabulary, according to the context where it's used.
やがる can be freely added to virtually any main verb in the language. Usage example: 話しやがる, which expresses a contemptuous tone that can be translated as "that idiot dares to talk". Here やがる is combined with the connective form of 話す.
Other advanced auxiliaries
The list of advanced auxiliary verbs that can be attached directly to the connective form doesn't end here. There are others, and now I'll quickly introduce a few more that you're likely to see in different contexts— some fairly common, some quite rare.
Note that these explanations are only meant to show the general function of each auxiliary. Whenever the word "action" appears, it refers to the action or state represented by the main verb to which the auxiliary is attached, but without necessarily reflecting the way the main verbs are used in terms of being transitive or intransitive.
● くさる (腐る): Expresses strong contempt for the doer of the action. Mainly used in the Kansai region, while the largely synonymous やがる is used in Kanto.
● くたびれる (草臥れる): To become exhausted by having continued to perform the action over a long period of time.
● くらす (暮らす): To perform the action all the time. Often has the nuance of "to lead a life" full of that action.
● こかす (転かす, 倒かす): To perform the action intensely and continuously. A similar auxiliary is こける.
● しめる (染める): To make a scent, color, etc. penetrate something by performing the action.
● すさむ (荒む): To perform the action violently, roughly, etc.
● そめる (初める): To begin to perform the action, particularly an action that takes a long period of time. This is mostly a literary word.
● ながす (流す): To perform the action cursorily, halfheartedly, evasively, etc.
● ならわす (習わす): To perform the action routinely, as a matter of habit.
● はぐれる (逸れる): To miss the chance to perform the action. Similar to そびれる.
● もらす (漏らす, 洩らす): To omit something while performing the action.
And one final tip
Most auxiliary verbs are also used as standalone verbs, and when you look them up in the dictionary you'll often get a confusingly long list of definitions.
If you want to quickly find out whether a verb is an auxiliary or not, go directly to the bottom of the list; monolingual Japanese dictionaries usually place the grammatical functions last, below all the lexical meanings. If you see a phrase like 動詞の連用形の下に付く, congratulations! You've got yourself a true auxiliary verb.
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