Meaning of "I didn't hear from him until now"

I understand that "I haven't heard from him until now" means "I didn't hear from him before and I just hear(d) from him now".

But what about "I didn't hear from him until now"? Does it mean the same as the previous sentence, or "I haven't heard from him ever, not even now"? Is it grammatical and does it sound natural?

By the way, can I replace "until now" with "till now" or "up to now" in those sentences? According to this Q/A, "until" is preferred for negative conditional sentences, but I am not sure if it's relevant to my question.

Answer

I'm not sure I have a definitive grammatical answer, but in my experience as a native American English speaker, the "didn't" variant is more correct and natural. Saying

I haven't heard from him.

implies that although you have not heard from him yet, there is some expectation you might hear from him in the future.

Whereas

I didn't hear from him.

is strictly talking about what is in the past. By using

I didn't hear from him until now.

you are adding contrast between the past and the present.

I suppose "I haven't heard from him until now" might be ok, but it seems like the "until now" (aha, something has changed!) conflicts with the hopefulness/tentativeness of "I haven't heard from him" — how can you still be hopeful if you already know that the situation has changed?

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re: "until now" vs. "till now" vs. "up to now":

"till" is an informal variant of "until". (although The Free Dictionary says that till came before until historically) If you say "up to now", people will understand you, and I don't think it is grammatically incorrect, but it is somewhat unnatural-sounding. I think the reason is that "until" is such a common preposition and has a very good match for this situation, that alternative forms don't seem right.

Hope this helps!