1) The third sentence would never be used. The first and second sentence are pretty much the exact same. Except: the 1st one would not be used when writing a paper (it also makes cops in TV shows look more badass for some reason :)). 2) both of the sentences are wrong. I'm not sure of your context, but I would probably say "don't get into TOO much detail" or "don't get into TOO many details". About your native speaker: everybody gets stuff wrong sometimes, even if they are natives :)

There really is no difference. Like at all. Except that you would use "I helped him..." If you were writing a paper.

It is definitely a slang word. It means like grungy, gross, not high-quality

Generally, you can use either one. But if you want to say something like "after lunch" or "after school got out" or "after we broke up", you would not use "afterwards".

There's not really a specific word for it that most people use. I just say whatever idk (I don't know :)). "Textspeak" is fine. My most common use of the word is when I tell my friends, "you know I don't know what you're weird text language means!"

When writing stories in English, descriptive sentences often go like this: "action, description". Action is: "Eric fell asleep". Description is: "his head gently tipping to one side". A comma between the action and description is required. You're translation is correct, but very unpoetic (sorry :)). It sounds much better and more interesting to not use the "while" and "was" you added.

1st sentence: it would be better to say "I'm off Thursday thru Saturday". ("I'm" is putting together "i" and "am", which is how native speakers actually talk. No one would say "I am" unless they were being very formal.)

Okay first, you would have to put "that" after "guess" in both of those sentences. "I guess that.." Or "I would guess that..." The two sentences pretty much mean the same thing, but you would use the 1st one a lot more. You'd only really use the second sentence if someone was like, "do you guess that this is true or false?" " I guess that this is true". You'd only use it if someone told you to guess an answer and you were responding. Sorry if that was confusing!