Mặc dù đã đăng ký, nhưng phải khảo sát.

Mặc dù means although / even though. It introduces a contrast.

So the basic idea is:

• mặc dù A, nhưng B
• although A, B

Here, the first fact is đã đăng ký and the contrasting fact is phải khảo sát.

That is a very common question. English and Vietnamese work differently here.

In Vietnamese, mặc dù ... nhưng ... is a normal contrast pattern. So even though English usually avoids although ... but ..., Vietnamese can use both markers together.

You may also hear:

• Mặc dù đã đăng ký, vẫn phải khảo sát.
• Dù đã đăng ký, vẫn phải khảo sát.

So nhưng is common, but not always required.

Đã is an aspect marker. Here it shows that the action of registering happened already, before the second action or requirement.

So:

• đăng ký = register / sign up
• đã đăng ký = have registered / already registered

It does not work exactly like an English past tense ending. Vietnamese often uses markers like đã, đang, and sẽ instead of changing the verb form.

Yes. Đã đăng ký rồi is also very common.

Compare:

• đã đăng ký = already registered
• đã đăng ký rồi = already registered, with a stronger sense of completion or emphasis

Adding rồi often makes the sentence feel a bit more conversational or emphatic.

For example:

• Mặc dù đã đăng ký rồi, nhưng vẫn phải khảo sát.

Yes. Vietnamese often leaves out the subject when it is clear from context.

So this sentence does not explicitly say who registered and who must do the survey. The subject might be understood as:

• bạn = you
• người tham gia = the participant
• người dùng = the user

English usually needs a subject, but Vietnamese can omit it much more freely.

Here, phải means must / have to / be required to.

So:

• phải khảo sát = must survey / have to do the survey

Be careful: phải can also mean correct / right in other contexts, but not here. In this sentence, it is a modal word showing obligation.

Often, yes. Many speakers would naturally say:

• Mặc dù đã đăng ký, nhưng vẫn phải khảo sát.

Here vẫn means still.

Without vẫn, the sentence is still understandable, but vẫn makes the contrast clearer and often sounds more natural:

• Even though already registered, still must...

So if you want a smoother, more typical sentence, adding vẫn is a good idea.

Khảo sát can mean different things depending on context, such as:

• survey
• inspect
• investigate
• study/examine

That means the sentence can be a little broad without more context.

For example:

• in an online form context, it may refer to a survey/questionnaire
• in a business or research context, it may mean conduct a survey
• in a technical context, it may mean inspect/examine something

So the exact English wording depends on the situation.

It is understandable, but it may sound a little incomplete depending on what you mean.

If the meaning is must conduct a survey / inspect something, then phải khảo sát can be fine.

But if the meaning is must fill out / complete a survey, Vietnamese often uses a fuller expression, such as:

• phải làm khảo sát
• phải tham gia khảo sát
• phải hoàn thành khảo sát
• phải trả lời khảo sát

So if the person is answering a questionnaire, phải khảo sát may not be the most natural choice.

A few natural possibilities are:

• Mặc dù đã đăng ký, nhưng vẫn phải khảo sát.
• Mặc dù đã đăng ký, vẫn phải khảo sát.
• Dù đã đăng ký rồi, vẫn phải làm khảo sát.
• Mặc dù đã đăng ký, nhưng vẫn phải hoàn thành khảo sát.

Which one is best depends on the exact meaning of khảo sát in your context.

If you mean take a survey, the last two are often clearer and more natural.