Learn Hardcore Vietnamese: Bữa tối hôm nay bắt đầu muộn vì tôi không có thời gian. - Dinner today starts late because I do not have time.

Bữa tối means “dinner” and hôm nay (“today”) modifies it to mean “today’s dinner,” i.e. “tonight’s dinner.” In Vietnamese, time modifiers often follow the noun. Saying Hôm nay bữa tối is grammatically possible but sounds less natural, and Tối nay alone means “this evening” rather than explicitly “dinner.”

What does bắt đầu muộn mean exactly, and can I use bắt đầu trễ instead?

Bắt đầu muộn literally means “start late.” You can say bắt đầu trễ (“start delayed”) to convey a similar idea, but muộn is more colloquial for a slight lateness. Trễ often implies a more serious or problematic delay.

What role does vì play in this sentence? Can I use bởi vì or nên instead?

Vì means “because” and introduces the reason clause. You can replace it with bởi vì (a bit more formal):
Bữa tối hôm nay bắt đầu muộn bởi vì tôi không có thời gian.
If you use nên (“so”) instead, you flip the order and express the result:
Tôi không có thời gian nên bữa tối hôm nay bắt đầu muộn.

What does không có thời gian mean? Is it different from không có đủ thời gian or bận?

Không có thời gian literally means “didn’t have time.”

Không có đủ thời gian adds đủ (“enough”) to stress “not enough time.”
Bận means “busy” and implies you lacked free time for that reason. You could say vì tôi bận to be more concise.

Can the subject tôi be omitted in vì tôi không có thời gian?

Yes. Vietnamese is a pro-drop language, so you can often omit pronouns when context is clear:
Bữa tối hôm nay bắt đầu muộn vì không có thời gian.
Including tôi simply adds clarity or emphasis on who was short of time.

Why isn’t there a future marker like sẽ for a dinner that hasn’t happened yet?

Vietnamese often uses the simple present tense with a time adverbial (hôm nay) to imply future events. Adding sẽ (“will”) is optional for emphasis:
Bữa tối hôm nay sẽ bắt đầu muộn.

What is the sentence structure? Can you break it down?

Subject: Bữa tối hôm nay (“tonight’s dinner”)
Predicate: bắt đầu muộn (“started late”)
Reason clause (adverbial): vì tôi không có thời gian (“because I didn’t have time”)

Is Bữa tối tối nay correct?

No. Bữa tối already means “dinner” and tối nay means “this evening.” Combining them as Bữa tối tối nay is redundant. Use Bữa tối hôm nay or simply Tối nay with the appropriate verb.