What sentence makes a contrast with (1)?
(1) He should be talking by now.
Do you think both work or that one works better than the other?
(2) a. He does not talk yet.
b. He has not talked yet.
Thank you in advance
Seiichi MYOGA
Hi
a. could work
but here are some other options
he isn´t talking yet.
he hasn´t spoken (his first words) yet.
Hi,
To add a bit more ’
a.
- could refer to a baby not old enough to talk
b.
- could refer to a lecturer/speaker/presenter who so far is not scheduled to talk until later.
Alan
Alan, you’re more erudite than I. I was picturing a supsect being interrogated by the police, not a lecturer!
Barb, I’m with you.
I immediately thought of a torture/interrogation suspect who hasn’t caved in yet. 
I watch too many spy thrillers, I guess…
As you can probably tell from the comments Seiichi, it depends on the context of the original statement. I’d prefer one over the other depending upon the situation being referred to.
Oh, I wasn’t thinking torture - just when you have the one suspect in one room, and the other in the other, and you tell one that the other has started telling all, so if they want their side of the story out, they better tell you… I watch too much Law & Order, I guess.
You’re not being interrogated by a the police. You’re being interrogated by someone who is doing formal research into syntax and semantics and is trying to get native speaker judgments without resorting to paid informants. Usually linguistic researchers pay people or use graduate students to do what he is asking us to do.