Hôm qua, tôi đã gặp một người bạn cũ ở công viên.

Hôm qua means yesterday. In Vietnamese, time expressions very often come near the beginning of the sentence to set the time frame first.

So this pattern is very common:

Time + subject + verb + ...

That said, you can also move it later:

Tôi đã gặp một người bạn cũ ở công viên hôm qua.

This is still correct, but beginning with Hôm qua sounds very natural and helps frame the event right away.

Đã marks that the action is viewed as completed or past. In this sentence, it helps show that the meeting happened before now.

However, Vietnamese does not have verb endings like English met, walked, or saw. The verb gặp stays the same. Instead, Vietnamese uses time words and particles like đã.

So:

• tôi gặp = I meet / I met / I have met, depending on context
• tôi đã gặp = I met / I had met / I did meet, depending on context

In this sentence, because Hôm qua already makes the time clear, đã is helpful but not absolutely required.

Yes. You can say:

Hôm qua, tôi gặp một người bạn cũ ở công viên.

This is still correct and natural because Hôm qua already tells you the event happened in the past.

Using đã adds a clearer past/completed sense. Omitting it can sound slightly simpler or more conversational.

So both are fine:

• Hôm qua, tôi đã gặp...
• Hôm qua, tôi gặp...

Gặp means to meet, to run into, or to encounter someone.

Thấy means to see.

That difference matters:

• gặp một người bạn cũ = met an old friend
• thấy một người bạn cũ = saw an old friend

If the idea is that you actually encountered the friend, gặp is the better choice. If you only noticed them from a distance, thấy would fit better.

Here, một usually works like a/an in English:

• một người bạn cũ = an old friend

Sometimes một really means one, but in many cases it simply introduces a singular noun naturally.

So in this sentence, it does not strongly emphasize the number one. It mainly means a.

Both bạn cũ and người bạn cũ can be correct.

• bạn cũ = old friend
• người bạn cũ = old friend, with a slightly fuller or more explicit noun phrase

Here, người bạn literally includes person/friend, but in practice it is just a natural way to say friend. It can sound a little more complete or slightly more careful than just bạn.

So:

• một bạn cũ sounds less natural
• một người bạn cũ sounds very natural
• bạn cũ is also common, especially when một is not used

No. In người bạn cũ, cũ means former, old from the past, or long-known, not elderly.

So this means:

• a friend you knew before
• an old friend from earlier in life

If you want to say the friend is old in age, you would need a different expression, such as:

• người bạn già
• người bạn lớn tuổi

So bạn cũ = old friend in the sense of former/longtime, not aged.

Ở công viên means at the park or in the park. It is a location phrase, and in Vietnamese, location phrases often come after the verb and object.

So this order is very natural:

tôi đã gặp một người bạn cũ ở công viên

That is:

• subject: tôi
• aspect marker: đã
• verb: gặp
• object: một người bạn cũ
• location: ở công viên

You can move location phrases for emphasis, but the sentence as given uses a very standard word order.

Ở often means at, in, or located at, depending on context.

In this sentence:

• ở công viên = at the park / in the park

Vietnamese does not always separate in and at as strictly as English does. The correct English translation depends on the situation, but ở is the normal word to mark location here.

A useful way to understand it is:

Time + subject + past/completion marker + verb + object + location

So:

• Hôm qua = time
• tôi = subject
• đã = past/completed marker
• gặp = verb
• một người bạn cũ = object
• ở công viên = location

This is a very common Vietnamese sentence pattern.

Not really. The comma after Hôm qua is optional and mostly a writing style choice.

So both are fine:

• Hôm qua, tôi đã gặp một người bạn cũ ở công viên.
• Hôm qua tôi đã gặp một người bạn cũ ở công viên.

In everyday writing, many people would simply write it without the comma.

Sometimes, yes, but only if the subject is already obvious from context.

For example, in a conversation, if everyone already knows you are talking about yourself, you might hear something like:

Hôm qua đã gặp một người bạn cũ ở công viên.

But as a full standalone sentence for learners, keeping tôi is clearer and more standard:

Hôm qua, tôi đã gặp một người bạn cũ ở công viên.

It is natural. A native speaker could absolutely say it.

That said, in casual speech, people often simplify slightly:

• Hôm qua tôi gặp một người bạn cũ ở công viên.

This sounds a little lighter because it drops đã, which is often unnecessary when Hôm qua is already present.

So the original sentence is correct and natural, but a slightly shorter version may sound more conversational.