I was wondering if you could please give us an update on the information the auditor was asking for ( related, regarding) (to, the) 199 tax returns for ABC.
The filing is due on May 15, 2019.
Both are fine, technically. However, "related to" would be my choice, because: 1) you already use a string of two progressive tenses ending in -ing; 2) "for" as a preposition would follow with another gerund verbal with -ing, butnin a different sense; so 3)"related to" is a better choice to break the rhythm and signal a clear break and change in focus. It's not a grammar problem but a style issue to help clarity in such a long sentence * I think you have apparently noticed since you're asking.
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Both are fine, technically.
However, "related to" would be my choice, because:
1) you already use a string of two progressive tenses ending in -ing;
2) "for" as a preposition would follow with another gerund verbal with -ing, butnin a different sense; so
3)"related to" is a better choice to break the rhythm and signal a clear break and change in focus.
It's not