Time words go first: Hôm nay tôi đi làm

In Vietnamese the time usually comes first, before the person: Hôm nay tôi đi làm. I'm going to work today. English tends to put the time at the end; Vietnamese puts it up front, where it sets the time for the whole sentence. That's one reason verbs don't need to change: Hôm qua tôi đi làm is "I went to work yesterday".

This works for any point in time: a day (thứ bảy), a part of the day (tối nay), a clock time (bảy giờ), or words like hôm qua and tuần sau. Questions keep the same order: Mai anh làm gì? What are you doing tomorrow?

No word is needed in front of the time. In particular, don't use ở before a day or a clock time: Bảy giờ tôi ăn sáng.

A time can also go at the end, usually with lúc before a clock time or vào before a day: Tôi ăn sáng lúc bảy giờ. That's an ordinary place for it too, and it fits well when the time is the answer to "when?". Words like mai, hôm qua and tuần sau take nothing in front at the end: Tôi đi Huế tuần sau. I'm going to Huế next week.

A length of time is different: hai tiếng or ba ngày goes after the verb.