What effect does the raven have on the speaker? Do you think that the speaker was already mentally unstable before the raven appeared? Or did the raven contribute to the speaker's mental deterioration?

The speaker had it in him, the way he started talking and expressing how he misses his lover showed that after she was gone he didn't realize what was going on around him. He also wasn't aware of his feelings too.

The arrival of the raven initially amuses the speaker, who is able to momentarily come out of his depression and smile. However, he quickly starts slipping back into his melancholy state. He feels intimidated by the bird that says "Nevermore" in response to his questions, and he suspects the raven of being a "thing of evil":

"Prophet!" said I, "thing of evil!-prophet still, if bird or devil!--

Whether Tempter sent, or whether tempest tossed thee here ashore,

For some reason, the speaker believes that the raven has the answers to all of his questions. However, he begins to get frustrated and starts yelling when he can't understand the meaning behind the raven's repeated "Nevermore."

Fancy unto fancy, thinking what this ominous bird of yore,-

What this grim, ungainly, ghastly, gaunt, and ominous bird of yore

Meant in croaking "Nevermore."

The raven has a frightening, maddening effect on the speaker. Even though he showed signs of insanity before the bird arrived, his sanity seems to deteriorate further when he believes that his soul is trapped in the raven's shadow:

And his eyes have all the seeming of a demon's that is dreaming

And the lamp-light o'er him streaming throws his shadows on the floor;

And my soul from out that shadow that lies floating on the floor