You're all caught up, so you aren't behind on anything

[kadioguy]

There is never enough time to do everything you have to do. You are literally swamped with work and personal responsibilities, endless e-mails, social media, projects, stacks of magazines to read, and piles of books you intend to get to one of these days—as soon as you get caught up.

[From the book Eat That Frog!]
---
Me: What does it mean by "get caught up"?
Friend: You're all caught up, so you aren't behind on anything.

Me: Why is it "behind on" rather than "behind"?
Friend: To be honest, I'm not sure. When you're talking about tasks, it's behind on. When you're talking about physically being behind someone, it's almost always behind.

I agree with him. However I'd also like to hear your opinions.

(The emphasis in colour is mine.)

 

[jutfrank]

I'd use behind with there.

You need a preposition there, whether it's on or with. In other words, the 'anything' (i.e., the task that you're behind with) is not the object of behind. Think of behind as more of an adjective than a preposition.

 

[Tarheel]

I'm okay with "behind on" there. (Maybe it's an AmE/BrE difference. I'm not sure.) Anyhow, the expression means you haven't been able to do everything you planned to do. (But you probably already know that.)

 

[emsr2d2]

I use both "behind on" and "behind with" with the same meaning.

 

[jutfrank]

Yes, behind on is fine for me too. I just meant that I'd naturally choose with.

 

[kadioguy]

One more item here: "behind in".

 

[Tarheel]

I'm not sure what the question is.

Is the question "Are all of those acceptable?" (Clearly, that source thinks they are.)

I would use "on" for the second one, but it is clear what is meant.

(Why do they "sth" instead of "something"? Do they think they are saving ink?)

 

[5jj]

Why do they "sth" instead of "something"? Do they think they are saving ink?

it's an extremely common space-saving abbreviation in dictionaries and grammars.