How we cite our quotes:
It pleased Ethan to have surprised a pair of lovers on the spot where he and Mattie had stood with such a thirst for each other in their hearts; but he felt a pang at the thought that these two need not hide their happiness. (4.26)
This passage shows that Ethan was aware that he was living a lie. This makes it all the more bewildering that he takes no steps to try to change his situation. Maybe it's not so bewildering. He's been told "no" about so many things, so many times that he hardly dares go beyond the simple pleasure of having Mattie's company.
She had an eye to see and an ear to hear: he could show her things and tell her things, and taste the bliss of feeling that all he imparted left long reverberations and echoes he could wake at will. (1.10)
This is a fancy way of saying that they Mattie not only listened to Ethan, but also understood him, and (this is big one) could talk to him. They have a lot in common.
The fact that he had no right to show his feelings, and thus provoke the expression of hers, made him attach a fantastic importance to every change in her look and tone. (2.38)
Not only are Ethan and Mattie hiding their love from others, but from each other as well. As long as nothing is said or done, they can pretend that they aren't doing anything wrong, and they aren't hurting Zeena.