When talking with a friend in Japanese, they said "行ってないです", they cannot
explain why it's different from "行きません/でした".  

As far as I'm aware, I'm not sure what the difference would be in terms of it
meaning "I did not go" or "I have not been".

I understand that the 'te' form can modify a sentence to extend it or
conjugate to てもいい etc, but in this context, what difference does it make
between the two? There's been a couple of times like this I haven't understood
the 'te' form fully. Any help would be much appreciated! :)

Thanks very much,  
Mat.
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2017-06-20T11:50:19.067
48562
2017-08-07T15:00:18.193
22626
question
grammar syntax politeness て-form
What is the difference between 行きます and 行ってきます (ました、ません etc)?
Great question!

行ってくる、or more commonly 行ってきます is actually a really common phrase. The most
common usage is when individuals leave their home, but I have (on rare
occasion) heard it used when people leave a gathering to go somewhere else.
(i.e. party to work) If you look at the translation on jisho.org, you will
find the [following
definition:](http://jisho.org/search/%E8%A1%8C%E3%81%A3%E3%81%A6%E3%81%8F%E3%82%8B)
`I'm off; see you later​.`

According to [this answer,](https://japanese.stackexchange.com/a/677/22352) we
find that 行ってきます has an implication of going, then coming back. This is why
you see it most commonly used when an individual leaves home for school/work.

行きます by contrast is simply translated as `I'm going.`

I want to make a note here: 行ってきます may seem to be related to the baseーて＋くる
grammar structure. Since the くる in 行ってきます is actually [implying
_motion,_](https://japanese.stackexchange.com/a/677/22352) it is **not**
related. Baseーて＋くる means: `came/got to be` I personally understand it as
`started to (verb),` but that translation has some issues. If you're
interested in learning about the Baseーて＋くる grammar, take a look at [this
question,](https://japanese.stackexchange.com/q/676/22352) as well as this
[website.](https://www.renshuu.org/grammar/82/%E3%81%A6%E3%81%8F%E3%82%8B) I
suggest that you take some time to study it when you get the chance.

Back on topic-- In short:

> 行ってきます
>
> **Literally:** I go and come back.
>
> **In natural English:** I'm off, see you later!

Finally:

> 行きます
>
> **Literally:** I'm going.
>
> **In natural English:** I'm going.
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2017-06-20T13:18:24.857
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2017-08-07T15:00:18.193
2020-06-17T08:18:27.500
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48562
answer