Question: How do you know whether to use dy- or dyi- at the begi

The short answer is to use what everyone else is using. ; )If you want a little more definitive answer for formal text, you can search for both words in the TLDC Corpus, and restrict the TDLC Corpus results to "All News Reports".Ex:
www.tagaloglessons.c om/examplefinder/ind ex.php?keyword=diyos ,dyosSome versions there is a clear correct answer, others they are used interchangeably.For example, for the term "newspaper", when you look at actual newspaper articles, they use both terms, but one is definitely more common:
www.tagaloglessons.c om/examplefinder/ind ex.php?keyword=diyar yo,dyaryo
diyaryo = 107x
dyaryo = 22x

@itsneiacard OMGulay...good on yah for thinking you can actually pronounce the difference at speed...Everyone develops their style...and you should too...decide which one you prefer or when...and enjoy your confidence.

@BoraMac I assumed she meant spelling, but maybe I’m wrong...

@jkos Oh, yes I did mean spelling hehe

@itsneiacard As a native speaker, we use the spelling "diy" for formal situations while "dy" is for informal. In speaking, you can hear some Tagalog speakers still using "diy" especially if they are from original Tagalog provinces like Batangas, Cavite and Bulacan.Example: Diyan = Dyan (Here)
Diyos = Dyos (God, but please note we don't use "Dyos" since God is always considered to be formal). I hope this helps you a bit