Learn Romanian: Radu întârzie; va fi în acel ambuteiaj. - Radu is late; he must be in that traffic jam.

This is the main point of this lesson! In Romanian, the future tense is often used to express a strong guess, deduction, or probability about a present situation. Since we know Radu is late right now, va fi means "he must be" or "he is probably" in the traffic jam at this very moment, not in the future.

Could I say în ambuteiajul acela instead of în acel ambuteiaj?

Yes, absolutely. Both mean "in that traffic jam". When the demonstrative goes before the noun (acel), the noun drops its definite article. When it goes after, the noun takes the definite article (ambuteiajul acela). Placing it before the noun is just slightly more emphatic.

Why is "is late" translated with just one word, întârzie?

In English, "late" is an adjective, so you use "to be" plus "late". In Romanian, a întârzia is a dedicated verb that means "to run late" or "to arrive late". So, întârzie literally means "he is running late", which covers the whole phrase without needing the verb a fi (to be).

Would it be wrong to use trebuie să fie instead of va fi here to mean "must be"?

Not at all! Trebuie să fie în acel ambuteiaj is perfectly correct and also means "he must be in that traffic jam". Using the future tense (va fi) is just another very natural, native-sounding way to express that same assumption without having to use a modal verb.