Doktor mengesan virus itu.

This sentence follows the common Malay pattern:

Subject + Verb + Object

• Doktor = subject
• mengesan = verb
• virus itu = object

So it works very much like English Doctor detects the virus, except that Malay handles things like articles and demonstratives differently.

In Malay, words like ini and itu usually come after the noun.

• virus itu = that virus or often the virus
• buku ini = this book

So English says that virus, but Malay says virus itu.

It can feel like either, depending on context.

• Literal meaning: that
• Natural translation in many sentences: the

So virus itu may mean:

• that virus if you are pointing it out or contrasting it
• the virus if both speaker and listener already know which virus is being discussed

Malay does not have a separate word exactly like English the, so itu often helps give that definite meaning.

Malay does not use articles the way English does. There is no direct equivalent of a/an or the in most sentences.

So:

• doktor can mean doctor, a doctor, or the doctor
• virus can mean virus, a virus, or the virus

Context tells you which one is meant. In this sentence, virus itu is more definite because of itu.

Mengesan means to detect, to trace, or to identify the presence of something, depending on context.

In a medical sentence like this, mengesan most naturally means detect.

Examples:

• Doktor mengesan virus itu = The doctor detects / detected the virus
• Alat itu mengesan asap = The device detects smoke

The base form is related to kesan, and the prefix meng- is added to form an active verb.

This is very common in Malay. The meN- family of prefixes often creates active verbs.

So:

• base/root: kesan
• active verb: mengesan = to detect

You do not usually say doktor kesan virus itu in standard Malay for this meaning. The prefixed form mengesan is the normal verb.

Malay verbs usually do not change form for tense. So mengesan by itself does not tell you whether the action is present, past, or sometimes even future.

The tense comes from context or from time words.

So this sentence could mean:

• The doctor detects the virus
• The doctor detected the virus

If you want to be more specific, you can add time words:

• Doktor sedang mengesan virus itu = The doctor is detecting the virus
• Doktor telah mengesan virus itu = The doctor has detected / detected the virus
• Doktor akan mengesan virus itu = The doctor will detect the virus

By default, it is usually understood as singular here: doctor or the doctor.

Malay nouns generally do not change form for singular vs. plural. If you want to make plural meaning clearer, you can use context or reduplication:

• doktor-doktor = doctors
• para doktor = doctors

So Doktor mengesan virus itu normally sounds like The doctor detected/detects the virus.

Yes, but it changes the feel slightly.

• Doktor mengesan virus itu = Doctor / The doctor detected the virus
• Seorang doktor mengesan virus itu = A doctor detected the virus

Seorang is a classifier used for one person, so it makes the a doctor / one doctor meaning more explicit.

Here it is capitalized because it is the first word of the sentence.

Malay capitalization works much like English in that respect. If the word appeared in the middle of a sentence and was not part of a name or title before a name, it would usually be lowercase:

• Saya bercakap dengan doktor itu = I spoke with the doctor

If it is used as a title with a name, then capitalization is normal:

• Doktor Ali
• Dr. Ali

A simple learner-friendly pronunciation is:

mengesan ≈ muh-nguh-san

A few useful notes:

• ng sounds like the ng in sing
• the e in me- is often a schwa, like the a in about
• stress in Malay is usually not as strong or dramatic as in English

So say it smoothly, without heavily stressing one syllable: mə-ŋe-san.

Yes. Malay often uses both active and passive patterns.

Active:

• Doktor mengesan virus itu = The doctor detected the virus

Passive:

• Virus itu dikesan oleh doktor = The virus was detected by the doctor

Here:

• dikesan is the passive form
• oleh doktor means by the doctor

Learners often meet both patterns, so it is useful to recognize them early.

Yes, very often.

In real Malay, noun + itu is frequently used where English would naturally say the + noun. It does not always sound as strongly pointing or distant as English that.

So in context:

• Doktor mengesan virus itu

may be best translated as:

• The doctor detected the virus

even though itu literally corresponds to that. Context decides how strong the that feeling is.