[kuyapz]

I am so sorry to bother all of you, but something is bothering me so much.

Could anyone explicate how is the following sentence is correct.

"Each of my friends was sick yesterday."

When do we use each of?

Thank you.

[susan53]

"Each of" would, I think, be very unusual in this context. If you are talking about two friends it would probably be :
Both of my friends were sick yesterday.
and for more than two
All of my friends were sick yesterday.

Both/all are used when you want to emphasise that something is the same for two or more people or things.
Each of is usually used to give the idea that the people/things should be considered individually because there is something different about each.

Compare :
He put all of the toys into a box - this probably means there was one box, ie all the toys went into the same box
He put each of the toys into a box - this probably means that there was a different box for each toy, ie each individual toy went into a different box.

I had dinner with all of my friends before I left - this probably means that there was one dinner and everybody attended it together.
I had dinner with each of my friends before I left - this would probably be used in the situation when there was a different dinner with each individual friend. So eg if there were 5 friends, he had dinner with the first on Monday, with the second on Tuesday and so on.


In your sentence it seems that you are saying that the problem was the same for both of them, and so both/all seems much more likely. You might use each however if you wanted to emphasise that they had different illnesses :

All of my friends were sick yesterday but each of them had something different : Robin had a bad cold, Chris had a stomach upset and Alex had a migraine

As always however, it's the choice of the speaker whether to consider the people/things as "the same" or "different". Here are some examples I found for each of in the British National Corpus :

He captures the peculiar oddity of each of them, and evokes their cities, their bars and their cultures.
She could have bought each of the boys a pair of trainers for what she had spent on the watercolours.
Screensport, the satellite television service, announced yesterday that it would be showing each of the World Series matches live.

In all of these examples, you could replace each with all or both, depending on the number involved, without changing the meaning.

And in fact, look at that last sentence :
In all of these examples, you could ...
I chose all of because I considered them as all the same (because they had the phrase "each of" in common). However, I could have seen them as three separate examples (because they all come from different sources). If I'd chosen that option, I would have said :
In each of these examples, you could ...

As always, grammar gives us the means of expressing different meanings, but it's up to us how we see the world and therefore which meaning we choose to express.

[kuyapz]

Thank you so much Miss Sue.

As always, you give simple, complete, and scholarly explanation.

Your students are so fortunate to have you.

Also, we are so blessed with your willingness to share.

I am wondering if you have written any books that we can buy or something...

[kuyapz]

I'm so sorry Miss Sue.
I found that you have already authored a lot of books. My bad.

[susan53]

Dear kuyapz,
No apologies needed. While I would love to be hugely famous, I'm afraid that I'm not. Oh well, in my next life perhaps..
Sue

[clevermae]

Each is a determiner. It is used before a singular noun and it is followed by a plural noun. In every single person you are appointing, it means each of your friends are sick.