Why Do I Pronounce Th as F?
Th-Fronting the Pronunciation of as a Sound in Words Like Thing or as a Sound in Words Like Brother Is a Characteristic Feature of London Speech Spreading...
Why do I say f instead of th?
So what the heck is it? Simply put, it's what happens when speakers use the sounds /f/ or /v/ instead of TH. This results in words like “thing” becoming “fing,” or “brother” becoming “bruvver” — and it can also make “three” and “free” sound identical.
Why can't I say my th?
Well, yes it could be normal that your child cannot produce the /th/ sound, depending on their age. ... Children acquire speech sounds in a particular order with some being early developing and others being later developing. /th/ is one of those later developing sounds.