Dù không thấy gian lận, tôi cũng khóa thẻ.

This is a very common Vietnamese pattern meaning even though / although ... still ...

So:

• Dù không thấy gian lận = Even though I didn’t see fraud
• tôi cũng khóa thẻ = I still locked the card

The word dù introduces a concession, and cũng in the main clause shows that the action still happened despite that condition.

A very literal breakdown is:

• dù = although / even if
• cũng = still / nevertheless

So the overall logic is:

Although X was not the case, I still did Y.

Yes, cũng often means also / too, but in the pattern dù ... cũng ..., it has a different function.

Here it means something closer to:

• still
• all the same
• nevertheless

So in this sentence, cũng does not mean also locked the card. It means:

Even though I didn’t see fraud, I still locked the card.

This is a good example of how Vietnamese words often depend heavily on sentence pattern, not just dictionary meaning.

Vietnamese often omits subjects when they are clear from context.

In this sentence, the subject of không thấy is understood to be the same as tôi in the main clause:

• Dù (tôi) không thấy gian lận, tôi cũng khóa thẻ.

So the full meaning is:

Even though I didn’t see fraud, I still locked the card.

This kind of omission is very natural in Vietnamese and happens a lot when the subject is obvious.

Not necessarily. Thấy can mean:

• see
• notice
• observe
• find / detect

In this sentence, không thấy gian lận is best understood as:

• didn’t see any fraud
• didn’t detect fraud
• didn’t notice fraudulent activity

So it is not just physical seeing. It often refers to recognizing or noticing something.

Gian lận usually means fraud, cheating, or deception, depending on context.

It can function like a noun:

• thấy gian lận = see fraud / detect fraud

It can also appear in verbal contexts:

• gian lận trong thi cử = cheat in exams
• gian lận tài chính = financial fraud

In your sentence, it works most naturally as a noun-like object of thấy.

Khóa means lock, and thẻ means card.

So khóa thẻ means:

• lock the card
• block the card
• freeze the card

In banking or payment contexts, English often says block the card or freeze the card, even though Vietnamese uses khóa.

So depending on context, the best English translation might be:

• I still locked the card
• I still blocked the card
• I still froze the card

Because khóa thẻ and hủy thẻ are different actions.

• khóa thẻ = temporarily lock/block/freeze the card
• hủy thẻ = cancel the card

If a bank suspects risk, it may khóa thẻ first. That does not necessarily mean the card is permanently canceled.

So this sentence suggests a protective action, not necessarily permanent cancellation.

Vietnamese does not mark tense the way English does. There is no mandatory past-tense ending.

So Dù không thấy gian lận, tôi cũng khóa thẻ could be understood from context as:

• Even though I didn’t see fraud, I still locked the card
• or less commonly, a general/habitual statement if the context supported it

In most real situations, this would be interpreted as past because the sequence of events sounds like a completed decision.

Vietnamese often relies on context, time words, or situation rather than verb inflections.

Yes. Mặc dù and dù are both used for although / even though.

For example:

• Mặc dù không thấy gian lận, tôi cũng khóa thẻ.
• Dù không thấy gian lận, tôi cũng khóa thẻ.

Both are natural.

A few notes:

• dù is shorter and very common.
• mặc dù can sound a bit more formal or explicit.
• In many cases, they are interchangeable.

The comma is very natural and helpful here because the sentence begins with a subordinate clause:

• Dù không thấy gian lận, = concessive clause
• tôi cũng khóa thẻ. = main clause

The comma makes the structure easier to read. In informal writing, people sometimes omit punctuation, but with this kind of sentence, the comma is standard and recommended.

Grammatically, dù can sometimes have an even if flavor, but in this sentence, the most natural reading is even though because of the overall context and lack of future marking.

If you wanted a clearly future or hypothetical meaning, Vietnamese would often add context such as:

• Dù không thấy gian lận, tôi vẫn sẽ khóa thẻ. = Even if I don’t see fraud, I will still lock the card.

So your original sentence is most naturally read as a real situation, not a hypothetical one.

Yes, and in many cases vẫn may sound even more direct for the meaning still.

Compare:

• Dù không thấy gian lận, tôi cũng khóa thẻ.
• Dù không thấy gian lận, tôi vẫn khóa thẻ.

Both are understandable and natural.

A rough distinction:

• cũng in this pattern works because of the structure dù ... cũng ...
• vẫn emphasizes continuity or persistence: still

In English, both may translate the same way. In real usage, many speakers might prefer vẫn here, but cũng is perfectly valid in the concessive pattern.

The structure is:

• Dù
  • subordinate clause
• main subject + cũng
  • verb + object

So:

• Dù không thấy gian lận
  = although not seeing fraud / although I didn’t detect fraud
• tôi cũng khóa thẻ
  = I still locked the card

A simple structural formula is:

Dù + clause A, subject + cũng + verb + object.

This is a very useful pattern to learn because it appears often in both speech and writing.