What is Dialogue?
Grammar and Spelling
"i used to be a adventurer like you but then i took an arrow to the knee."
Then again, seeing that phrase at all, good grammar or not, is annoying.
If something isn't your native language, I recommend running everything through a program that can check for both spelling and grammar. And even if it is your native language, I still recommend doing that.
Act Natural
Tailoring Your Text
Noble/academic: Oh, yes. I'm quite familiar with Jorrvaskr. I live in the Cloud District, as you likely know. You can find Jorrvaskr there, too.
Farmer: Eh? Oh, yeah. Jorrvaskr's up there, by the palace. Strange place, if you ask me. Up in the Cloud District, where all those fancy nobles strut about.
Guard: Yes, citizen? Oh. Well, Jorrvaskr's in the Cloud District. To see the way the Companions fight... It's crazy.
Senile man: Hmmm. Up you go. To Jorrvaskr, it's where they are. Yes, that's where it is. Mmmhmm. When I was young, I fought for the Companions.
It's All in the Face
"They killed my grandmother right in front of me."
If a voice actor does a fantastic job, then it's the modder's job to capitalize on that, and make the dialogue as good as possible. For that line, I would give it a Sad facial expression, or a Disgusted facial expression, with a power of about 80. (Once you start creating responses, you'll know what I mean.)
Be Concise, Be Clear
Remember the Player
"They killed my grandmother right in front of me."
May be something along the lines of "I'm so sorry, that's horrible" keep in mind that the player using your mod may feel something completely opposite ("Well, everybody dies, some just die quicker" if they're more sadistic or pragmatic). Thus, it's best to give at least a few general options that could apply to more playstyles. This will make it far less likely to break immersion and make the player think "But I wouldn't say that!"