When to use "il faut", and when to use "il faut QUE"?

"​Falloir" is a very peculiar verb in French: the main thing to remember is that it can only be used in the impersonal form: "il faut". You cannot say: "je faus, tu faus, on faut" etc...
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"Il faut" is easy to use therefore when in a general context, meaning 'one', 'we' or 'people' must...
​e.g. Il faut fermer la bouche quand on mange. (One must close one's mouth when one eats.)​
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​However, it gets trickier when you need to express 'I, you, he, she, [specific]we... must': that is when you use "Il faut que" + the right person + verb in Le Subjonctif.
​e.g. Il faut que je sois à l'heure demain. (I must be on time tomorrow.)
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I hope that's helpful.

Very good explanation! You can also use an indirect object pronoun, e.g., Il me faut être à l'heure.