Hearthpwn

If I dredge a card and then do a draw, then I am guranteed to get the card that I dredged because draw means drawing from the top of your deck.

But what if I dredge a demon and then use Sense Demons which draws only demons? does it gurantee to draw the card that I dredged? what if I dredge a non demon? how does this work?

sorry if this was answered before

Tutor effects draw random cards of that type, which may include the topmost ones but not always. Tutors don't shuffle as well, which means if you Dredge a Siphon Soul then play Sense Demons, your next draw is going to be Siphon Soul. 

What does interrupt Dredge are cards which shuffle others into your deck, as well as ones which reorder your deck. If you Dredge a Doomguard and then your opponent plays Helya, your next draw is no longer guaranteed to be Doomguard. Lorekeeper Polkelt also denies that Doomguard Dredge, making you draw your Mal'Ganis instead. 

Yes, if you dredge and then draw you draw the dredged card, as long as you don't shuffle things into your deck before drawing.  Think of it like Gone Fishin', even if you are using two cards to dredge and then draw, the effect is the same as when you combo Fishin'.

As DatMageDoe stated, tutors like Sense Demons randomly pull cards of a certain type regardless of order.  This shouldn't effect the Dredge unless it randomly pulls the demon you dredged.  So if you dredge Doomguard it COULD get pulled by Sense Demons, making your next draw unknown.  However, if it doesn't pull Doomguard, it would be your next draw.

In that specific scenario, Sense Demons (or any tutor draw) followed by the Dredge might be the better order most of the time.