Bài này được viết bởi bác sĩ.

Bài is a very flexible noun. Depending on context, it can mean lesson, exercise, article, piece of writing, post, or text.

So bài này means this lesson/article/text/piece, depending on what kind of bài you are talking about.

In this sentence, if the meaning shown to the learner is something like This article was written by a doctor, then bài is being understood as article or piece of writing.

In Vietnamese, demonstratives like this and that usually come after the noun.

So:

• bài này = this article/lesson
• bác sĩ này = this doctor

This is different from English, where this comes before the noun.

A very common pattern is:

noun + này = this + noun
noun + đó / kia = that + noun

Here, được is helping form a passive sentence.

So in Bài này được viết bởi bác sĩ, được viết means something like is written / was written.

Be careful: được can mean different things in different sentences, such as:

• to get / receive
• to be allowed to
• a passive marker

In this sentence, it is the passive marker.

Yes. This is a passive sentence.

The basic idea is:

Bài này = the thing receiving the action
được viết = is written / was written
bởi bác sĩ = by a doctor / by the doctor

So the structure is roughly:

thing + được + verb + bởi/do + agent

In active voice, the same idea would be:

Bác sĩ viết bài này.
The doctor wrote/is writing this article.

Yes. In this sentence, bởi means by, introducing the doer of the action:

• được viết bởi bác sĩ = written by a doctor

However, learners should know that bởi is often a bit more formal or bookish in this kind of sentence. Vietnamese very often uses do instead:

• Bài này được viết do bác sĩ. — less natural
• Bài này do bác sĩ viết. — much more natural
• Bài này được viết bởi bác sĩ. — grammatical, formal

Also, do not confuse this bởi with bởi vì, which means because.

It is grammatical, but it sounds somewhat formal and a little stiff in everyday speech.

More natural alternatives are often:

• Bài này do bác sĩ viết.
• Bác sĩ viết bài này.

So if you are learning conversational Vietnamese, you will probably hear do or a simple active sentence more often than được ... bởi.

In standard Vietnamese, that sounds incomplete or unnatural.

If you want a passive structure, được helps mark it clearly:

• Bài này được viết bởi bác sĩ.

If you do not want to use được, a more natural option is usually:

• Bài này do bác sĩ viết.

Or you can switch to active voice:

• Bác sĩ viết bài này.

So for learners, it is safest to remember:

• được + verb for a clear passive
• do + agent + verb as a common alternative

Vietnamese does not use articles like English a, an, and the.

So bác sĩ can mean:

• a doctor
• the doctor
• sometimes even doctors, depending on context

The surrounding context tells you which one is meant.

If Vietnamese wants to be more specific, it can add other words, for example:

• một bác sĩ = a doctor
• vị bác sĩ đó = that doctor
• bác sĩ này = this doctor

But in many sentences, just bác sĩ is enough.

By itself, viết does not show tense.

Vietnamese verbs do not change form the way English verbs do. The time is usually understood from:

• context
• time words
• markers such as đã, đang, sẽ

So được viết could mean:

• is written
• was written
• sometimes even has been written

depending on context.

If you want to make the time clearer, you could say:

• Bài này đã được viết bởi bác sĩ. = This article was written by a doctor / has been written by a doctor.
• Bài này sẽ được viết bởi bác sĩ. = This article will be written by a doctor.

Both can appear in passive-like sentences, but they have different feelings.

• được is usually neutral or positive
• bị is usually used for something negative, unwanted, or unfortunate

Examples:

• Bài này được viết bởi bác sĩ. = neutral/positive
• Bài này bị sửa quá nhiều. = This article got edited too much / negative feeling

So you would not normally use bị here unless you wanted some negative meaning, which does not fit written by a doctor.

Usually, bác sĩ means doctor in the medical sense.

In Vietnamese, it does not normally mean a person with a PhD in the broad English sense. So in this sentence, most learners should understand bác sĩ as a medical doctor.

If you wanted doctor as an academic title, Vietnamese would usually express that differently depending on the context.

The word order is:

Bài này + được viết + bởi bác sĩ

So, piece by piece:

• Bài này = this article / this lesson
• được viết = is written / was written
• bởi bác sĩ = by a doctor

A useful pattern to remember is:

subject/thing + được + verb + bởi/do + doer

Example:

• Cuốn sách này được viết bởi tác giả nổi tiếng.
• This book was written by a famous author.

That pattern will help you recognize many Vietnamese passive sentences.