Hi! "Have to" means obligation. Number 1 means you have to because your boss said so for instance. Number 2 means you have to because the doctor said you have to for instance. What about number 3? You heard the pizza at the restaurant is good. How is that an obligation? Thanks, Fulvio
1. I have to attend that meeting.
2. I have to eat vegetables.
3. I have to eat that pizza.
yellowstarstruck 3. I have to eat that pizza. = My appetite is forcing me to eat it .
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yellowstarstruck3. I have to eat that pizza.
= My appetite is forcing me to eat it.
However, keep in mind that native speakers do not always make a clear distinction between 'must' and 'have to'. The rule you have learnt is a guideline.