"Will have done" vs "would have done" in expressing certainty

Question

Recently, I have been looking a little bit deeper into the modal verb “will”, its usage and properties. In the process I have been able to find the usage I have already encountered, though have seen very few times: some people will use “will” to imply certainty. However, seemingly, there is not much information about it on the Internet (at least, not much have I managed to find). So, I am here for the answers, as well as to share some thoughts I have got and ask you to dispute them in case I should have the wrong conception.

According to Dictionary.com, one of the meanings, functions of the verb “will” is to express certainty.

1. You will know. (I think you know.)
2. You will have seen me before. (I think you have seen me before.)
3. It will be John at the door (I think it is John at the door)

If at this point everything might seem obvious, I have questions concerning two following sentences.

4. They will have heard the news

5. They would have heard the news

Both, basically, describe the past; nonetheless, I presume there is a considerable difference regarding time references, just as in the old good Past Simple and Present Perfect. “Will have heard” sounds good in the example, “Martha is calling; she will have heard the news”, whereas “would have heard” sounds good in “Martha called me out of the blue three times yesterday; she would've heard the news”

Despite all of my observations above, I have seen people use those interchangeably. One could say that something "will have happened yesterday". In my turn, I wouldn't say it is the best choice, but I am a non-native speaker and not dictating the rules, which apparently has led me to the forum, so I would love to hear you thoughts.

My next query is about “would” and “would + past p.”

I conjecture in some contexts, they are interchangeable, and in another not. (We are still talking about this wonderful property of “will” to describe certainty, do not confuse with conditionals)

Again, I shall share my thoughts and I shall wait for yours.

I think sentences:

6. “He was late yesterday; his mom would have been pissed”
7. “He was late yesterday; his mom would be pissed”

are interchangeable. (Yet, the second might be better, because the first uses more formal grammar which does not look natural given the context) Although, the same would not work with the verb “take” for example.

8. “It was raining; she would've taken an umbrella”;

I would think “she would take” is not correct in this case.

Last but not least, I have never seen such structre used in question, negative, continuous forms.

9. Will they have heard the news?
10. They wouldn't have seen the moon;sky wasn't clear yesterday.
11. The smell of fresh pastry was floating all over the kitchen; Mom would've been baking something

Do the sentences above have sense?

In any way, I thank you for having read all of this. If any of my examples are just not idiomatic or do not have sense, please let me now. In the end, it is the point of this post.

Thank you. I will be delighted to read your comments

Answer 1

You clearly understand that modal will+infinitive can be used to express certainty or confidence about a present or future situation. And that will+have+past participle can be used to express certainty or confidence about a past situation.

On this basis, all of your example sentences can be expressed with will:

Present

• You will know (how I feel).
• It will be John at the door.

Past

• You will have seen me before.
• They will have heard the news.
• Martha called me yesterday; she will have heard the news.
• He was late yesterday; his mom will have been pissed.
• It was raining; she will have taken an umbrella.
• Will they have heard the news?
• They won't have seen the moon; the sky wasn't clear.
• Mom will have been baking something.

In all of the above sentences will can be replaced by would, with no change in meaning - except, perhaps, that would might be considered to express very slightly less certainty or confidence.

Your only claim that is problematic is that sentences 6. and 7. are interchangeable.

He was late yesterday; his mom will/would have been pissed means that the mom was annoyed yesterday by his lateness.

He was late yesterday; his mom would be pissed sounds unnatural to me, although it could conceivably express a certainty about the mom's continuing and present state of annoyance. But if this is the case, then will is better:

• He was late yesterday; his mom will be pissed.