Tôi không nhớ hôm qua tôi đã để chìa khóa ở đâu.

Because the sentence has two parts:

• Tôi không nhớ = I don't remember
• hôm qua tôi đã để chìa khóa ở đâu = where I left the key(s) yesterday

Vietnamese often repeats the subject in the second clause, even when it is the same person. To an English speaker, the second tôi may feel unnecessary, but in Vietnamese it sounds natural and clear.

Here đã marks a completed action in the past. In this sentence, it shows that the action of leaving/putting the keys happened before now.

It is not always exactly the same as English already. In many cases, đã is better understood as a past or perfective marker. A rough literal breakdown is:

• tôi đã để = I left / I had put

If you really want to emphasize already, Vietnamese often uses rồi instead, depending on context.

Also, because hôm qua already shows past time, đã is helpful but not absolutely required in casual speech. Still, the full sentence with đã sounds very natural.

In this sentence, để means to leave, to put, or to place somewhere.

So để chìa khóa ở đâu means leave/put the key(s) where.

This verb has several different uses in Vietnamese, so learners often get confused by it. In other contexts, để can also mean things like:

• to let
• to allow
• in order to

But here it is simply the verb leave/put.

Because Vietnamese keeps normal word order in both direct and indirect questions.

• Direct question: Tôi đã để chìa khóa ở đâu? = Where did I leave the keys?
• Indirect question: Tôi không nhớ ... tôi đã để chìa khóa ở đâu. = I don't remember where I left the keys.

Unlike English, Vietnamese does not move where to the front of the clause. The location phrase stays where it would normally appear.

So ở đâu literally means at where, and it stays after the verb phrase.

Because the sentence is asking about a location.

• ở = at / in
• đâu = where

So ở đâu means where in the sense of at what place.

In some contexts, đâu can appear by itself, but ở đâu is the most straightforward and natural form when asking where something is or where something was left.

It can mean either key or keys, depending on context.

Vietnamese nouns usually do not have to show singular or plural the way English nouns do. So chìa khóa by itself is neutral about number unless something else makes it clear.

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

• một chìa khóa = one key
• những/các chìa khóa = keys

In the original sentence, the exact number is not the main point, so chìa khóa works naturally.

Because classifiers are not always required.

Vietnamese often uses classifiers when counting, specifying, or pointing out individual objects, but they can be omitted when the noun is being mentioned more generally.

So:

• một cái chìa khóa = one key
• chìa khóa = key / keys, in a general sense

In this sentence, the speaker is talking about the lost item without focusing on counting it, so leaving out the classifier is normal.

Because không is the standard negation word used before verbs and adjectives in Vietnamese.

So:

• nhớ = remember
• không nhớ = do not remember

This is very typical Vietnamese word order:

• Tôi không biết = I don't know
• Tôi không hiểu = I don't understand
• Tôi không nhớ = I don't remember

English uses do not before the verb; Vietnamese uses không directly before the verb.

In the original sentence, hôm qua belongs to the second clause, the action of leaving the keys:

• hôm qua tôi đã để chìa khóa ở đâu = where I left the keys yesterday

That placement makes it clear that yesterday tells us when the keys were left.

This matters because changing the position can slightly change the meaning. For example:

• Tôi không nhớ hôm qua tôi đã để chìa khóa ở đâu.
  = I don't remember where I left the keys yesterday.

• Hôm qua tôi không nhớ tôi đã để chìa khóa ở đâu.
  = Yesterday, I didn't remember where I had left the keys.

Those are close, but not identical. The original sentence focuses on the leaving happening yesterday.

Sometimes in casual speech, Vietnamese can omit a repeated subject if it is very clear from context. But in a sentence like this, keeping the second tôi is the safest and most natural choice.

So learners should usually say:

• Tôi không nhớ hôm qua tôi đã để chìa khóa ở đâu.

rather than trying to shorten it too early. The repeated subject makes the sentence clearer and more standard.

Yes. A helpful rough breakdown is:

• Tôi = I
• không = not
• nhớ = remember
• hôm qua = yesterday
• tôi = I
• đã = past/completed action marker
• để = leave/put
• chìa khóa = key(s)
• ở đâu = where

So a very literal reading is:

I not remember yesterday I had left key(s) where.

That is not good English, of course, but it shows how Vietnamese builds the sentence.