Họ hứa sẽ khảo sát.

Họ usually means they/them in Vietnamese.

A few useful notes:

• It is a third-person plural pronoun.
• It does not show gender, so it can mean they for any group.
• Depending on context, họ can sometimes also mean people in a more general sense, but in this sentence it most naturally means they.

Yes. This follows the normal Vietnamese pattern:

Subject + Verb + future marker + main verb

So here:

• Họ = subject
• hứa = promise
• sẽ = will
• khảo sát = survey / investigate / conduct a survey

Vietnamese word order is often similar to English in simple statements, so They promise will survey maps fairly closely to They promise they will survey.

Hứa means to promise.

It is commonly followed by:

• a verb phrase: hứa sẽ làm = promise to do / promise that [someone] will do
• or an indirect object: hứa với ai = promise someone

In this sentence:

• hứa sẽ khảo sát = promise to carry out a survey / promise that they will investigate

Vietnamese often leaves out words that English needs, so there is no separate that here.

Yes, sẽ is the common marker for future meaning, similar to will.

Here it shows that the action of khảo sát happens after the promise:

• hứa sẽ khảo sát = promise that they will survey/investigate

A useful point:

• With hứa, the future idea is already somewhat implied, so sometimes sẽ could be omitted in casual or context-clear speech.
• But including sẽ is very natural and makes the future meaning explicit.

Yes, it can, depending on context.

For example:

• Họ hứa khảo sát.

This can still be understood as They promise to survey / investigate.

However:

• Họ hứa sẽ khảo sát sounds more explicit about the future action.
• Sẽ helps make the sentence clearer and often more natural in careful speech or writing.

Khảo sát has a range of meanings depending on context. Common translations include:

• to survey
• to examine
• to investigate
• to conduct a survey
• to inspect in some contexts

Its exact meaning depends on what is being surveyed or studied.

For example:

• khảo sát ý kiến người dân = survey public opinion
• khảo sát hiện trường = inspect/examine the scene
• khảo sát thị trường = survey the market

So in your sentence, without more context, survey or investigate are both possible.

In this sentence, khảo sát is being used as a verb.

Vietnamese words are often flexible, and many can function as more than one part of speech depending on position.

Here:

• after sẽ, it is clearly verbal: will survey / will investigate

In other contexts, it can also appear in noun-like expressions, for example:

• cuộc khảo sát = a survey
• bản khảo sát = survey form / survey document

So the word itself is flexible, but here it is definitely verbal.

Vietnamese often leaves out information that is understood from context.

So Họ hứa sẽ khảo sát is grammatically complete even without stating the object. It means something like:

• They promise they will carry out a survey
• They promise they will investigate

If you want to be more specific, you can add the object:

• Họ hứa sẽ khảo sát khu vực này. = They promise they will survey this area.
• Họ hứa sẽ khảo sát ý kiến người dân. = They promise they will survey residents’ opinions.

Yes, you can think of one as being understood.

English often says:

• They promise that they will survey...

Vietnamese usually does not need a word equivalent to that in this kind of sentence:

• Họ hứa sẽ khảo sát.

Sometimes Vietnamese can use words like rằng in more formal styles, but it is not necessary here.

So a natural way to understand the structure is:

• Họ hứa [rằng] sẽ khảo sát.

A rough pronunciation guide is:

• Họ ≈ haw with a heavy low tone
• hứa ≈ something like hoo-uh with a broken/rising quality
• sẽ ≈ seh with a creaky/rising tone
• khảo sát ≈ khow ssaht with tones that native pronunciation will matter for

A few pronunciation notes:

• kh is not like English k alone; it is a breathy sound similar to the ch in German Bach for many learners.
• Vietnamese tones are essential, so even if the consonants and vowels are close, the tones matter a lot.
• khảo sát has two syllables: khảo
  • sát.

If you want a natural rhythm, the sentence is spoken in four clear chunks:

• Họ | hứa | sẽ | khảo sát

Yes, native speakers could definitely say it, but it sounds a bit neutral to formal, especially because of khảo sát.

Why:

• hứa sẽ... is completely normal
• khảo sát is a somewhat formal or official-sounding verb

So this sentence feels natural in contexts like:

• news
• government statements
• business or research
• official announcements

In casual everyday speech, people might choose a simpler verb depending on meaning, but Họ hứa sẽ khảo sát is still perfectly good Vietnamese.

Yes. Vietnamese pronouns depend heavily on relationship, age, and social context.

So instead of họ, you might hear:

• anh ấy hứa sẽ khảo sát = he promises he will survey/investigate
• cô ấy hứa sẽ khảo sát = she promises she will survey/investigate
• bên họ hứa sẽ khảo sát = their side promised they would investigate
• người ta hứa sẽ khảo sát = they/people promised they would investigate

Using họ is fine when you simply mean they and do not need a more specific social label.