To ask a general question such as "what are you doing?" which verb would one use?

Sen ne etiyorsun?
Sen ne yapıyorsun?

Ne yapıyorsun? is better.
Ne ediyorsun is also acceptable but not common. I hear that mostly in countryside, or from the people who grew up in little centers/cities or countrysides. Sometimes I use it while I am speaking with my close friends as a joke/slang:
"ne ediyon"="n'ediyon"
"n'örüyon"

By the way they can be used as a slang something like "whazzup?" or "how're you doing?" or "just "how are you?"

and in addition "what's up" and "what are the news" slangs;
"n'aber"(we know it mostly as "naber"= "ne haber")
"n'ettin"(ne yaptın=ne ettin)
But they are not for formal things, if you are very close, you can use them. Normally: Ne yapıyorsun(uz)?(+uz for mr.brown who is not close)

d to t changes apply when the suffix begins with a vowel I think.

If there is more than one suffix that changes de and den.
eg. arabası -dan becomes
arabasından
Hope this is right as I am only a beginner but think I have remembered correctly
Cheers Jo-Anne

I am still not understanding why et-iyor-sun becomes ediyorsun :-S
I understand consonant harmony but this lesson says:

However, if the word has more than one syllable, than the consonant at the end changes.
- p becomes b
- ç becomes c
- t becomes d
- k becomes ğ

et is only one sylable so should not be subject to the change from t to d!

Yes bod,you are right

Like bakmak....it is still bakıyorum ..K didn't change into Ğ here

But as in English there are some irregulars,or exceptions from the rules

And with ( yor ) there are lots of exceptions too

Like the negative suffix (ma) has 4 shapes not only 2

So it is a rule also 't' change into 'd' if it was followed by vowel even if it was 1 syllable

As in gitmek too it is 1 syllable and it changed to gidiyor

So always T changed to D in any case

That is what i know, correction please if it was wrong ,Ty