Back and again can be used with similar meanings, but there are some differences.
With a verb, we use back to suggest a return to an earlier situation, a movement in the opposite direction to an earlier movement, and similar ideas.
Again is not normally used in this way with a verb.
Give me my watch back. (NOT give eme my watch again.)
I’m taking this meat back to the shop. (NOT I’m taking this meat to the shop again.)
With a verb, again usually suggests repetition. Compare:
That was lovely. Can you play it again?
When I’ve recorded your voice I’ll play it back.
Eric was really bad-mannered. I’m never going to invite him again.
She comes to our parties but she never invites us back.
I don’t think he got your letter. You’d better write again.
If I write to you, will you write back?
Note the difference between sell back (to the same person) and sell again.
The bike you sold me is too small. Can I sell it back to you?
If we buy this house and then have to move somewhere else, how easy will it be to sell it again?
When the verb itself already expresses the idea of ‘return to an earlier situation’ or ‘movement in the opposite direction’, back is not generally used.
Stefan can never return to his country. (More natural than Stefan can never return back … )
Who opened the window? Could you close it, please? (NOT … close it back– … )
However, again can be used to emphasize the idea of ‘return’.
Stefan can never return to his country again.
Who opened the window? Could you close it again, please?
With adverb particles and prepositional phrases, we can use both back and again to suggest ‘return to an earlier situation’ etc.
I stood up, and then I sat (back) down (again).
He tasted the apple and spat it (back) out (again).
Go (back) to sleep (again).
I’ll be (back) in the office (again) on Monday.
Note that ring back (BrE only) and call back can be used to mean both ‘return a phone call’ and ‘repeat a phone call’.
She’s not here just noui.r-Ask her to ring me back. (= return my call)
I haven’t got time to talk now. ~ OK, I’ll ring back later. (= ring again)
Back is an adverb particle, and can usually go between a verb and its object, unless this is a pronoun Again cannot.
Take back your money – I don’t want it. (OR Take your money back … )
Count the money again, please. (NOT
count again the money … )