وقتی دوباره همدیگر را دیدیم، هر دو لبخند زدیم.

وقتی means when. It introduces a time clause:

• وقتی دوباره همدیگر را دیدیم = when we saw each other again

In Persian, it is very common to begin a sentence with a when-clause like this, followed by the main clause:

• وقتی ... ، ...
• When ..., ...

So the overall structure is:

• وقتی دوباره همدیگر را دیدیم = when we saw each other again
• هر دو لبخند زدیم = both of us smiled

دوباره means again.

So:

• دوباره دیدیم = saw again
• وقتی دوباره همدیگر را دیدیم = when we saw each other again

It tells you that this was not the first meeting.

همدیگر means each other or one another.

So:

• همدیگر را دیدیم = we saw each other

This word is used when the action is reciprocal, meaning both sides do it to one another.

You may also see:

• یکدیگر = another, slightly more formal/literary way to say each other

In everyday Persian, همدیگر is very common.

را marks a specific direct object.

In this sentence:

• همدیگر را دیدیم

the verb دیدیم means we saw, and همدیگر is the object: each other.
Because it is a definite/specific object, Persian uses را.

So the pattern is:

• X را دیدن = to see X
• او را دیدم = I saw him/her
• همدیگر را دیدیم = we saw each other

English does not have a separate marker like this, so it can feel unusual at first.

Persian is usually a Subject–Object–Verb language, so the verb often comes at the end of the clause.

Compare:

• English: we saw each other again
• Persian: we each other again saw

That is why you get:

• همدیگر را دیدیم
• literally: each other saw-we

The same thing happens in the second clause:

• لبخند زدیم
• literally: smile struck-we
• natural English: we smiled

Persian often drops subject pronouns when the verb ending already shows the subject.

Both verbs here end in -یم, which marks we:

• دیدیم = we saw
• زدیم = we hit/struck or, in this expression, we smiled

So Persian does not need to say ما explicitly.

You could say ما for emphasis, but it is not necessary:

• وقتی دوباره همدیگر را دیدیم، هر دو لبخند زدیم
• perfectly natural without ما

هر دو means both.

In this sentence it means both of us:

• هر دو لبخند زدیم = we both smiled

Since the verb ending already tells us the subject is we, هر دو naturally means both of us, not just both in some abstract sense.

You can use هر دو with nouns too:

• هر دو دوست = both friends
• هر دو کتاب = both books

This is a very common Persian pattern called a light verb construction.

• لبخند = smile
• زدن = literally to hit/strike, but in many expressions it helps form idiomatic verbs

So:

• لبخند زدن = to smile
• لبخند زدیم = we smiled

Many everyday Persian actions are expressed this way:

• حرف زدن = to talk
• نگاه کردن = to look
• تصمیم گرفتن = to decide

So even though زدن literally means to hit, here it is just part of the normal expression for smile.

Yes, if you translate word-for-word, it can sound odd:

• لبخند = smile
• زدیم = we struck/hit

But Persian uses light verbs very naturally, and learners should usually treat لبخند زدن as a whole unit meaning to smile.

It is similar to how English says things like:

• take a look
• have a rest
• make a decision

The individual verbs do not always keep their full literal meaning.

Both are in the simple past.

• دیدیم = we saw
• زدیم = we smiled

So the sentence describes two completed actions in the past:

1. we saw each other again
2. we both smiled

Because the first clause is introduced by وقتی, English often translates it with when.

The verb ending tells you.

• زدیم ends in -یم, which means we
• so هر دو must refer to both of us

If it were both of them smiled, the verb would be third person plural, for example:

• هر دو لبخند زدند = both of them smiled

So the verb ending is the key clue.

The comma is helpful and very natural, because it separates the time clause from the main clause:

• وقتی دوباره همدیگر را دیدیم، هر دو لبخند زدیم.

It works like English:

• When we saw each other again, we both smiled.

In informal writing, punctuation in Persian can sometimes be looser, but this comma is perfectly standard.

A natural pronunciation would be approximately:

vaqti do-bâre ham-digar râ didim, har do labkhand zadim

A few notes:

• وقتی = vaqti
• دوباره = dobâre
• همدیگر = hamdigar
• را after a vowel often sounds like ro in colloquial speech, but in careful reading it is râ
• لبخند = labkhand
• زدیم = zadim

A more colloquial spoken version might sound a bit like:

vaqti dobare hamdige-ro didim, har do labkhand zadim

Yes. A common alternative is همو? No, not in standard formal Persian. The main standard alternatives are:

• همدیگر = each other
• یکدیگر = each other, more formal/literary

So you could also say:

• وقتی دوباره یکدیگر را دیدیم، هر دو لبخند زدیم.

This sounds a bit more formal or written.
For everyday speech, همدیگر is more common.

It is mostly neutral natural Persian.

Why?

• وقتی is standard and common
• دوباره is common
• همدیگر را دیدیم is normal and natural
• لبخند زدیم is standard everyday Persian

A very colloquial spoken version might change pronunciation or wording a little, but the sentence as written is perfectly normal and natural in standard Persian.