Or is that "you"?

You, because there's よ.

I understand all the words but not the meaning of this sentence. Would you mind elaborating ?

The Japanese sentence means "You often peek in other people's cellphones when they're using them. You should stop that habit."

Can this English perhaps also mean "You can peek in other people's cellphones, but you should stop peeking when they're using them; you should peek in their cellphones only when they're not using them"?

I still don't understand how one can peek in a cellphone while the owner uses it...does it mean "hacking" the phone ??

Mais ce n'est pas difficile ...
http://www.photolibrary.jp/img1...6_1218549.html

Perhaps it's a reference to confidential information that might be displayed on a screen. Maybe it's just something like "Kehre for der eignen Tür"— literally "Sweep in front of your own door," but idiomatically, according to my Cassell's bilingual dictionary, "Mind your own business."

How about:
"You tend to look at the display screen when people are using their cellphones. Please stop doing that."

Do you think this comes close to the meaning of the Japanese text, tommy_san?

@Objectivesea

> "You tend to look at the display screen when people are using their cellphones. Please stop doing that."

Yes, you're basically right.
Actually, mine is one sentence, so something like this:

"You'd better stop the habit of looking at the display when people are using their cellphones."

Do we have to say "display" in order to be understood?

Or screen ? It would definitely help understand this sentence, in my opinion...

Some phones are equipped so you can't see the display content if you look at it from an angle. Screens can also be fitted with sticking tape which you can cover the screen with and provides the same kind of protection.
Anyway, you've got to have a bloody good eyesight to see phone screens content from behind someone's shoulder !

These capture the nuance.

* You should quit looking at what other people are doing on their cellphones.
* It's impolite to look at other people's cellphone screens when they are doing private stuff.
* Watching other people's cellphone screens while they are writing email isn't considered good manners.

覗き込む doesn't always have a negative meaning, right?
https://www.google.com/search?q...2%80s&tbm=isch

> You should quit looking at what other people are doing on their cellphones.

I like this one, and I think it can be linked to my sentence.
We sometimes use adverbial clauses when we translate noun clauses in Western languages, so you can use noun clauses when you translate Japanese adverbial clauses.