have to
hafta
/ˈhæftə/AssimilationReduction
In real speech
What happens to the sounds
The v before the voiceless t turns into f, and to weakens to /ə/: have to → “HAF-ta”.
When to use it
Almost everyone says it this way, even in formal conversation. It's still written have to.
Casual → formal
Slang
hafta
Neutral
have to
Formal
must / am required to
Sound notes
Assimilation: neighbouring sounds change each other, e.g. t + y → “ch”, d + y → “j”.