Learn Dutch: Hij komt vast morgen. - He is definitely coming tomorrow.

In Dutch, you can use the simple present tense to talk about the future, especially when a specific time like morgen (tomorrow) is included. You do not need to use a future tense word like zal (will).

I thought vast meant "stuck" or "fixed". Why does it mean "definitely" here?

You are right that vast often means "stuck" or "fixed". However, when used as a small descriptive word in a sentence like this, it means "surely", "certainly", or "definitely". You can think of it as the plan being "fixed" and certain to happen.

Could I say "Hij is morgen aan het komen" to match the English "is coming"?

No, Dutch does not use the continuous aan het form for future events. The aan het structure is only used for actions happening right at this very moment. For future plans, just use the simple present tense.

Why does vast come before the time word morgen?

Small words that express certainty (like vast, zeker, or misschien) usually come very early in the sentence, right after the main verb. Because of this, vast comes before the time word morgen.