In the most terrible and nimble stroke

Regan basically blinds Gloucester. Then Regan regrets blinding Gloucester because allowing him to live arouses sympathy which results in more parties turned against Regan and her company.

Of quick cross lightening to watch, poor perdu

Edgar leads Gloucester to Dover and pretends they are walking up the steep hill Gloucester wished to be taken to. Edgar says that it is steep and he can hear the ocean, noting that Gloucester's other senses must have grown dim as well if he cannot feel these things. Gloucester comments that poor Tom's speech seems much more elevated than before so Edgar attempts to drop back into his beggarman dialect. Edgar says they have reached the highest spot and Gloucester asks to be placed where he is standing. He then takes out another purse for Tom and requests to be left. Thinking Tom has gone, Gloucester prays to the gods to bless Edgar and then wishes the world farewell and falls forward of the cliff, he believes. Edgar approaches again as another man entirely, playing along with the idea that Gloucester has fallen off the high cliff and survived, calling it a miracle. Gloucester believes what the man says, though he cannot look up to verify. Edgar helps him up and questions the thing which left him at the top of the cliff, making it sound like it was not an actual man but a spirit. Gloucester is skeptical at first but realizes that would make sense for why he lived.

With this thin helm? Mine enemy's dog,

Though he had bit me, should have stood the night

The audience had not heard any sort of passion from the earlier Cordelia but we hear her now, and the change is extremely welcomed by the audience.

SparkNotes: King Lear: Act 3, scenes 1–3

Cordelia thanks Kent for the goodness he has shown her father and the bravery he has espoused. She asks him to discard his disguise but he knows that he will be able to work better for Lear if he remains disguised. The remarks that Lear has slept for a long while so that they may try waking him. Lear is brought in, still sleeping. Hoping to resolve the horrors committed by her sisters, Cordelia kisses Lear and reflects on the vileness and ingratitude of her sisters, treating Lear worse than a dog by shutting their doors on him in the storm. Lear wakes and Cordelia addresses him. Lear feels awakened from the grave and wishes they had left him. Very drowsy at first, Lear thinks Cordelia is a spirit and then realizes he should know her and Kent (disguised) but has difficulty putting his memory together. Finally he recognizes Cordelia, to her delight, but thinks he is in France. The Doctor advises them to give Lear his space so Cordelia takes him for a walk. The gentleman remains and asks Kent if the rumors of Cornwall's death and Edgar's position in Germany with the are true. Kent confirms the first, but leaves the latter unanswered. The gentleman warns that the battle to come will be bloody.

The Tragedy of King Lear :|: Open Source Shakespeare

Act IV begins on a misleading high note as Edgar is pleased that any changes in his life will have to bring better times. Things cannot get worse, he implies. The paradox is established then with Gloucester's subsequent entrance and Edgar realizes that his life has gotten worse now that he knows the terrible treatment his father has endured. It is important to keep in mind that Edgar does not know how Edmund deceived his father into believing Edgar was the evil doer. All Edgar knows is that he had to run for his life because of the feelings Gloucester, Cornwall, and Albany held against him. Yet, even though he is incredibly saddened by Gloucester's appearance and torment, he does not once act reluctant to aid his father.