Hễ nhập đúng mật mã là đồng hồ đếm ngược dừng.

Hễ introduces a condition that happens whenever or as soon as something is true.

In this sentence, Hễ nhập đúng mật mã là... means something like:

• Whenever the correct password is entered, ...
• As soon as the correct password is entered, ...

It often suggests a very regular or automatic result.

No. Here, là does not mean the normal verb to be.

In the pattern hễ ... là ..., là helps introduce the result clause:

• Hễ A là B = Whenever/if A happens, then B happens

So in this sentence:

• Hễ nhập đúng mật mã = if/whenever the correct password is entered
• là đồng hồ đếm ngược dừng = then the countdown timer stops

You can think of this là as functioning a bit like then, not is.

They are similar, but not exactly the same.

• nếu ... thì ... = if ... then ... (neutral conditional)
• hễ ... là ... = whenever / as soon as ... then ... (often more automatic, repeated, or immediate)

So:

• Nếu nhập đúng mật mã thì đồng hồ đếm ngược dừng.
  = If the correct password is entered, the countdown stops.

• Hễ nhập đúng mật mã là đồng hồ đếm ngược dừng.
  = Whenever / as soon as the correct password is entered, the countdown stops.

The version with hễ feels more like a built-in rule or automatic response.

Vietnamese often leaves out the subject when it is obvious, generic, or not important.

Here, nhập đúng mật mã literally means enter the correct password, but in natural English we often make it passive or more general:

• when the correct password is entered
• if you enter the correct password

The sentence focuses on the condition, not on who performs the action.

Here it is best understood as correct describing the password:

• nhập đúng mật mã = enter the correct password

Even though word-for-word it may look like enter correctly password, Vietnamese often places đúng before the noun in this kind of expression to mean that the thing entered is the right one.

So the natural interpretation is:

• nhập đúng mật mã = enter the right password/code

Mật mã can mean code, secret code, or sometimes password, depending on context.

In a device/security context, it can often be translated as:

• password
• passcode
• access code

So in this sentence, mật mã is likely the code/password needed to stop the countdown.

Literally:

• đồng hồ = clock / timer
• đếm ngược = count down

So đồng hồ đếm ngược means:

• countdown timer
• literally, countdown clock

In English, countdown timer is usually the most natural translation.

Dừng means stop and is perfectly natural here.

Some close alternatives are:

• dừng = stop
• dừng lại = stop, come to a stop
• ngừng = cease, stop

For a machine or timer, all of these may be possible in some contexts, but dừng is short and natural.

So:

• đồng hồ đếm ngược dừng = the countdown timer stops

You may also hear:

• đồng hồ đếm ngược dừng lại
• đồng hồ đếm ngược ngừng

But the original sentence is fine.

Vietnamese often does not mark tense explicitly if the meaning is clear from context.

This sentence expresses a general rule or automatic result, so English may translate it as:

• the countdown timer stops
• the countdown timer will stop

Both can work depending on context.

Vietnamese relies heavily on context rather than verb endings or fixed tense forms.

Yes, it is understandable and fairly natural, especially in instructional, technical, or dramatic contexts.

A few common alternative phrasings would be:

• Hễ nhập đúng mật mã là đồng hồ đếm ngược sẽ dừng.
• Nếu nhập đúng mật mã thì đồng hồ đếm ngược sẽ dừng.
• Nhập đúng mật mã thì đồng hồ đếm ngược dừng.

Adding sẽ makes the result feel more explicitly future:

• sẽ dừng = will stop

But the original sentence is already grammatical and clear.

Yes. That is a very good natural translation.

Depending on context, possible English renderings include:

• As soon as you enter the correct password, the countdown stops.
• If the correct password is entered, the countdown stops.
• Whenever the correct password is entered, the countdown timer stops.

The best choice depends on whether you want to emphasize:

• immediacy → as soon as
• condition → if
• repeated rule → whenever

The structure is:

• Hễ + condition + là + result

Applied here:

• Hễ = whenever / as soon as
• nhập đúng mật mã = enter the correct password
• là = then
• đồng hồ đếm ngược dừng = the countdown timer stops

So the sentence works as:

Whenever/as soon as [the correct password is entered], [the countdown timer stops].