In the words like "splatter", "splash", "split" and "explain" should I make an aspirated or an unaspirated "p" sound? I can't decide which way is correct. I need your help.
In most varieties of English, voiceless plosives (/p/, /t/, /k/) are aspirated when they are the whole of the onset of a syllable, but not when they are in a cluster at the onset of a syllable.
So /p/ is not aspirated in any of your examples. It is aspirated in "patter" (but not "platter" or "spatter") and "passion", "pit", and "impound".
For me (British English), it is not normally aspirated in "expand" though, and I think that is because I effectively pronounce it as "ek-spand"