"Why did she go without me?"
Translation:Mengapa dia pergi tanpa saya?
"Mengapa dia pergi tanpa saya?" ... saya lapor deh :)
(Translation: "Mengapa dia pergi tanpa saya?" ... I'll report [this] :) )
 "deh" is an interjection which I think doesn't have an English equivalent.
EDIT (2020-05-27): Looking back at this a year later, I have absolutely no idea what I was reporting and why, haha. Probably because it wasn't accepted back then? Now, though, it seems to be the default answer. Nice.
Sorry for the late reply :( I don't think I got the email back then... or maybe I didn't see it. My bad.
"deh" in the sense of "saya lapor deh" is a lot like "I guess I'll just..."
So "saya lapor deh" would be like "I'll just report this."
In other contexts (especially combined with an affirmative, as in "iya deh" ["iya" means "yes"] or "oke deh" ["oke" is "okay"]), it could also be something like "okay then" or "alright then"... I think the context is usually when agreeing with someone you previously didn't agree with, or fulfilling a request you previously didn't want to fulfill.
Oh, another one: in compliments (or insults, or opinions in general), I think it would correspond to "you know?" as in "kamu keren deh" (you're cool, y'know?)... in this particular context, a colloquial variation of "deh" is "dah" (not that "deh" isn't colloquial though, haha)...
but as for opinions, it would correspond to "I think...", so for example, "kayaknya [itu] kebanyakan deh" ("kayaknya" is colloquial for "sepertinya", while the colloquial affix pair "ke-an" corresponds to the formal "terlalu", not to be confused with the formal affix pair "ke-an" which would correspond to the English suffix -ness), which would mean "I think that [it/that is] too much"
I'm not sure if that was an exhaustive list, haha, but I couldn't think of any other possibilities off the top of my head as of right now. I might come back with more examples (probably not, I guess that's all).
EDIT (2020-05-27, an hour later on the same day):
Yeah, another context: to soften an imperative, or at least a suggestion to try doing something... as for the softened imperative, here's an example: "baca ini deh" (as in "hey, read this") (in a soft tone)... while a suggestion to try doing something would be of the form "coba ... deh" ("coba" means "try", here it acts as an imperative), so using the same example: "coba baca ini deh" (as in "hey, try reading this")... practically it conveys the same meaning, although I would argue that the suggestion is a little softer than the softened imperative.
As an Indonesian, I can confirm this.
If necessary, you could use words like "sudah" (colloquial: "udah" or even "dah") (meaning: already) to emulate the present perfect, "tadi" (meaning: earlier) to emulate the past simple, and... that's about it for emulating the past tense, I guess.
If you need the continuous aspect, you can use "sedang" (I'm not sure how to translate this, probably something like "currently"), or colloquially, "lagi" (in dictionaries, and colloquially as well, "lagi" means "already", but in what I'd like to call "colloquial Indonesian", some words can take on multiple unexpected meanings. It doesn't make sense, I know, but that's just the way it is, and us Indonesians live with it and use it without much thought.)
Yes, "mengapa" is formal.
Colloquially, just about everyone uses "kenapa". It sounds formal or awkward if you use "mengapa" in a casual conversation.
However, in our homework, exams, or in debates, I think interviews, and maybe some other examples I couldn't think of off the top of my head, we use "mengapa", because using "kenapa" in those situations would sound too informal.