Meaning of verb+となる


> ところが、引き返す **となったとたん** 、痛みがましてきた。  
>  However, just as it became _time to_ go back, the pain increased still
> further.

I don't understand the grammar of the part in bold. My translation is a best
guess.

I'm familiar with the use of とたん to mean 'just as', and なる to mean 'become',
but I don't know any grammar where と+なる can attach to a verb.




I think this 引き返す is a case of zero-nominalization (discussed here: [Zero-
nominalisation - Why and
When?](https://japanese.stackexchange.com/questions/4489/zero-nominalisation-
why-and-when)).

This means that it works, against all appearances, as a noun phrase, and is
part of the familiar construction "NP + と + なる" ("become NP").

Hence " **引き返す** となったとたん" more or less equals "as soon as [it] became **that
[I'm] going back** ". (I'm trying to come up with a natural translation in
context but keep drawing a blank.)

Whereas the parallel of "it became **time** to go back" would be "引き返す **時**
に/となった".




Your translation is correct. As for the +となる part, allow me to explain:

Normally, when something becomes something, normally you'd see +になる.

However, using +となる implies a finality, as in having FINALLY become that stage
of change.

The translation might become more correct if it were: "However, just as it
**ultimately** became time to go back, the pain increased still further."

...or something to that extent. Hope this helps!

