He said twice over before he knew himself: "Can't a man speak of his own child he's lost?" (36-37)
The man feels that, for him, the subject of his dead child is taboo. Also note how he edges around the concept of "dead" without actually saying the word "dead." This is something that happens often in this couple's communications. No one wants to say "dead," probably because it somehow makes the death more real.
Her fingers moved the latch for all reply. (47)
The woman in this poem does a lot of nonverbal communicating. Instead of replying to her husband with words, she'll just do something subtle, like stiffen her neck, or move the latch of the door. This could mean a couple of things: (1) she's so sick of not being able to communicate with her husband that she feels he's not even worthy of words, or (2) that she's dismissing him without actually coming right out and saying it.
"I don't know how to speak of anything
So as to please you. […]" (49-50)
This couple's conversation is so stilted that the man feels that he can't speak to his wife at all. These lines put all the blame on her, though we have a feeling that he's partly at fault here, too. In the rest of the poem, he shows us how he pretends to try to listen to and support his wife without making much of a real effort at all.
"You can't because you don't know how. (75)
The wife is putting her hubs down hard here, telling him he doesn't know how to speak, much less about this child. He could speak about his dead child, but certainly wouldn't know the right way, according to her. Basically, he's doomed from the get-go.
"Think of it, talk like that at such a time!" (98)
In this line, the woman reacts to what she believes is yet another instance of how her husband doesn't know how to speak. Right after digging his son's grave, the man talks about birch fences rotting. Now, we can see that rotting actually does have something to do with graves. But if we were this woman, that's definitely not talk that we'd want to hear after watching our son's grave be dug.
"You—oh, you think the talk is all. […]" (116)
The wife says this line just after her husband tells her to talk about her grief. If only it worked that way. For this lady, grief is more than just talk. It's buried deep down inside, beyond the reach of language and communication. And beyond repair.