Learn Hardcore Vietnamese: Tôi đang viết một lá thư cho khách mới.

đang indicates that the action (writing the letter) is happening right now. It’s the Vietnamese way of expressing the continuous or progressive tense, similar to the English "am writing."

What role does một play here?

một means "one" or "a/an." In Vietnamese, when you say một lá thư, you’re specifically indicating that it’s one letter. Native speakers often use một to give a sense of "a single item" or "an item" in this context.

Why is lá used before thư?

lá is the classifier for flat or thin objects (like leaves, sheets of paper, letters, etc.). Since thư (letter) is a flat object, Vietnamese grammar requires using lá to classify it whenever you want to be precise.

Can we say Tôi viết một lá thư without đang?

Yes, but it switches the focus from "I am writing (right now)" to a more general statement "I write a letter" (could be a habit or a one-time future action). Including đang emphasizes the ongoing aspect of the activity.

Why don’t we say khách mới của tôi instead of khách mới?