# Why is 一緒に correct and 一緒で incorrect?

This is a mistake I habitually make, even though I know what the right form is.

Earlier tonight, I wanted to tell my friend "Let's go together!", so I sent a mail saying:

My friend sent back a mail :

はい、一緒{いっしょ}に行{い}こう ;)

... reminding me that で is incorrect, and に is the appropriate particle in this case.

Which I know is right, but the reason it's stuck in my head to use で is that it makes more sense to me.

に is used to indicate direction of action(?), so I don't see how "going" can be directed at "together".

で makes sense to me, because my understanding of で is that it means "by use of" or "by way of" or something like that.

So to me it makes perfect sense to say 一緒{いっしょ}で行{い}こう because with で it means "let's go, and the way we'll go is together".

Can someone help me break this habit by making some kind of logical sense of why に is right and で isn't?

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First, we can't make the blanket statement that 一緒で is always incorrect, only that it is incorrect in this particular case.

Let's start by identifying how に is used here. In the case of 一緒に, に works the same way as in expressions like きれいに ("neatly", "prettily") and 気軽に ("casually"): it turns the preceding word into an adverb and shows how the action is done.

Because the point of 一緒 in the sentence 一緒に行こう is to show how the action is done ("together" as opposed to, say, "separately"), it must be followed by に.

On the other hand, で, when following a quantity or a noun identifying a group, expresses the range of the subject (such as in 一人で, 二人で, みんなで, and 家族で) and shows how many people/things participated in the action. 一緒 is too vague to be used with で in this sense, because it does not specify a number or a precise grouping.

But there are cases when 一緒 can be followed by で. This typically indicates that the parties involved are in the same physical location, but they may not be performing the same action. (In these cases, the line between the particle で and the ～て form of だ gets blurry.)

この会社では、社員は一緒で昼食をとる。 At this company, the employees have lunch together. [Implying that some may start eating a little before others, but everyone has lunch in the same place and at roughly the same time.] (incorrect)

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I think that この会社では、社員は一緒で昼食をとる。 is simply incorrect. Other than that, I agree with what you stated in this answer. –  Tsuyoshi Ito Jul 16 '11 at 21:45
@Tsuyoshi: Hmm, can you elaborate on why it's incorrect? Is it a grammatical error, or just unnatural? –  Derek Schaab Jul 17 '11 at 0:25

So, in a sense, would it be accurate to think of 一緒に as doing something "togetherly"? –  Questioner Jul 18 '11 at 3:48
@Dave M G: "togetherly" sounds good to me. (And I'm tempted to start using "togetherly" just to see the reactions I'd get. :) –  Derek Schaab Jul 19 '11 at 13:20

一緒に: adverb. Which means, it gives a description to an action (verb).

Went to the beach together with my little sister.

The verb 行く now has extra information: it was done together 妹.

一緒で: predicate + conjuction. Which means, 一緒 is used to describe that the subject is together. Then で is used to continue description with other predicate (description).

To borrow Derek's examples:

1. 友達と一緒でよかった。

becomes

and

1. 子供と一緒で荷物が多いと大変です。

becomes

(note: this split sentences are talking about self. The original joined sentences could be a general statement.)

and

becomes

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This is only a speculation, but the etymology might be relevant. 一緒に derives from 一所に, which means 'at/to one place'. So the expression 一緒に行こう 'let's go together' is etymologically driven from 一所に行こう 'let's go to a single place'. In the latter, 一所 'a single place' is the destination, and hence you should use に instead of で. If that is just carried over to the usage of 一緒に, then it will make sense.

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just to confirm you are saying that the kanji for issho has changed after it was introduced into the japanese language? –  Mark Hosang Jul 17 '11 at 5:02
@Mark Yes. Most proably by folk etymology. –  user458 Jul 17 '11 at 11:50