lost on you/rearranging to send/sending

Hi, please tell me:

  1. the meaning of expression: lost on you

  2. which one sentence is correct and:(…/…):

a)I had been rearranging sending one letter to Russia for some/longer time, but finally/eventually didn’t do it.

b)I had been rearranging to send one letter to Russia for some/longer time, but finally/eventually didn’t do it.

Thank You very much

  1. If something is said to be ‘lost on someone’, then it means either they don’t understand it or they don’t appreciate it.

be lost on somebody
to be not understood or noticed by somebody
His jokes were completely lost on most of the students.
oxfordlearnersdictionaries.c … #lost__148

idioms.thefreedictionary.com/lost+on

The sentences don’t make sense as written. For a start, I think that you have confused ‘rearranging’ with ‘postponing’/‘putting off’/‘delaying’ - and if you didn’t do it at all, then ‘but’ (indicating an opposing idea) is not suitable:

I had delayed sending a letter to Russia for some time, but finally I did it.
I had delayed sending a letter to Russia for a long time and eventually I didn’t do it at all.

Thank You Bees, but why there is Past Perfect and not Past Parfect Continuous in the second sentence?

Because:

I had delayed - a single, short, finished action
I had been delaying …for some time/a long time - it lasted this period of time

It’s a completed action (it finished when either the letter was eventually sent or when the decision was made that the letter would never be sent).

But, I mean that the delaying was repeated during/for some time, for example every week until I didn’t do it.

You would not count that as delaying week by week. It’s not the same as a series of (different) delays. You would count it as one delay from the time the delay started until it ended.

In my view, ‘I had been delaying … .’ is also possible as contended by Saneta in terms of the length of time. It could have been a deliberate action until it was decided otherwise. The delay, therefore, did continue or was going on for some time.

Possible, but not as likely/natural. ‘Been’ is unnecessary there and the simpler form of the verb is generally recommended where that is the case, unless there is a specific reason for using the complex form.