Tôi đã chọn quà rồi, nhưng tôi vẫn đang nghĩ xem nên viết gì trong thiệp.

Both đã and rồi can show that an action is completed, but they do it slightly differently.

• đã marks the action as happening in the past or being completed.
• rồi adds the sense of already.

So Tôi đã chọn quà rồi feels like:

• I already chose the gift
• I’ve picked out the gift already

Using both together is very common and sounds natural. It emphasizes that this part is finished.

You could also hear:

• Tôi đã chọn quà.
• Tôi chọn quà rồi.

These are also natural, but đã ... rồi is a very common completed-action pattern.

vẫn means still, and đang marks an action as in progress, like am/is/are ...-ing.

So:

• vẫn = still
• đang nghĩ = am thinking
• vẫn đang nghĩ = am still thinking

This combination is very common in Vietnamese.

So nhưng tôi vẫn đang nghĩ means:

• but I’m still thinking
• but I’m still in the middle of thinking about it

It shows contrast:

• the gift has already been chosen
• but the message for the card has not been decided yet

In this sentence, xem does not literally mean to see/watch. After verbs like nghĩ, it often helps create the sense of considering or thinking about what/whether to do.

So nghĩ xem is like:

• think about
• consider
• figure out

Here:

• nghĩ xem nên viết gì = thinking about what I should write

This use of xem makes the sentence sound natural and conversational. It softens the idea and gives a sense of mentally considering options.

Vietnamese question words usually stay in the position where the answer would go. This is very different from English.

In English, you say:

• What should I write?

The what moves to the front.

In Vietnamese, the question word stays in place:

• nên viết gì
• literally: should write what

This is called an in-place question word pattern.

Other examples:

• Bạn ăn gì? = What are you eating?
• Anh ấy đi đâu? = Where is he going?
• Cô ấy gặp ai? = Who did she meet?

So viết gì is completely normal Vietnamese word order.

nên usually means should, ought to, or it would be good to.

In nên viết gì, it means:

• what I should write
• what would be good to write

It suggests choosing the best or most appropriate thing to say in the card.

So the phrase is not just about physically writing something. It is about deciding what is suitable.

Vietnamese often leaves out subjects when they are already clear from context.

In this sentence:

• Tôi đã chọn quà rồi
• nhưng tôi vẫn đang nghĩ xem nên viết gì trong thiệp

The subject of nên viết gì is obviously the same tôi, so it does not need to be repeated.

You could say xem tôi nên viết gì, and it would be understandable, but it is less natural here. Vietnamese often prefers the shorter version when the subject is already obvious.

This is a subtle but useful point.

• trong thiệp = in the card
• trên thiệp = on the card

When talking about the message you write as part of a greeting card, trong thiệp is very natural because you are thinking of the written message inside the card.

trên thiệp could be used in some contexts, especially if you mean text physically written on the surface of the card, but trong thiệp is the more natural choice for what to write in a card.

Vietnamese often leaves out classifiers and words like một when speaking generally or when the exact count is not important.

So:

• chọn quà = choose a gift / pick out the gift / choose gifts, depending on context

If you want to be more specific, you could say:

• chọn một món quà = choose a gift
• chọn món quà = choose the gift

In the original sentence, chọn quà sounds natural because the main point is simply that the gift-buying decision is already done.

Yes, thiệp usually means a card, especially a greeting card, invitation card, holiday card, birthday card, and so on.

Depending on context, you may also hear more specific forms:

• thiệp chúc mừng = greeting card / congratulatory card
• thiệp sinh nhật = birthday card
• thiệp cưới = wedding invitation/card

In this sentence, just thiệp is enough because the context already makes it clear.

Yes. The original sentence is natural, but there are simpler variations.

For example:

• Tôi chọn quà rồi, nhưng tôi vẫn đang nghĩ nên viết gì trong thiệp.
• Tôi đã chọn quà, nhưng vẫn đang nghĩ nên viết gì trong thiệp.

These still sound natural.

The original version is slightly fuller and smoother because:

• đã ... rồi strongly marks completion
• nghĩ xem adds the feeling of considering possibilities

So the original sentence sounds very natural and nicely conversational, but it is not the only possible way to say it.