Odată ce se termină setul, schimbăm perechea de dublu.

Here odată ce means once or as soon as.

So Odată ce se termină setul means something like Once the set ends or As soon as the set is over.

A useful thing to know is that odată ce can also mean now that / given that in other contexts, but in this sentence it has a clear time meaning.

Because a se termina means to end / to come to an end / to be over.

In Romanian:

• a termina ceva = to finish something
• a se termina = to end / to finish

So:

• Terminăm setul = We finish the set
• Setul se termină = The set ends / The set is over

In your sentence, the set is not actively finishing something else, so Romanian uses se termină.

The se here is part of the verb pattern and often gives this intransitive or middle meaning. It is very common in Romanian.

The subject is setul.

So the structure is:

• se termină = ends / is ending
• setul = the set

Even though setul comes after the verb, it is still the thing that is ending.

Romanian often allows this kind of word order, especially when introducing what is happening rather than who is doing it.

Because setul means the set.

Romanian usually puts the definite article at the end of the noun:

• set = a set
• setul = the set

Here the speaker means a specific set, probably the one currently being played, so the definite form is natural.

Because Romanian often drops subject pronouns when the verb ending already makes the subject clear.

• schimbăm = we change / we switch

The ending -ăm already tells you the subject is we, so noi is unnecessary unless you want emphasis.

So:

• schimbăm = normal, natural
• noi schimbăm = we change, with extra emphasis

Yes, it is grammatically present tense.

Romanian often uses the present tense for:

• habitual actions
• instructions
• scheduled actions
• things that happen whenever a condition is met

So Odată ce se termină setul, schimbăm perechea de dublu can mean something like:

• Once the set ends, we switch the doubles pair
• When the set is over, we change the doubles pair

This is very similar to English, which also often uses present forms after time words like once and when.

It means the doubles pair or the pair for doubles.

Breaking it down:

• perechea = the pair
• de dublu = for doubles / in doubles

In sports contexts, dublu is the standard term for doubles. So pereche de dublu is a normal way to talk about a team of two players playing in a doubles event.

Depending on context, a natural English translation could be:

• the doubles pair
• the doubles team
• the pair playing doubles

Because dublu is the usual Romanian sports term for the doubles format or event.

For example, Romanian uses expressions like:

• meci de dublu = doubles match
• probă de dublu = doubles event
• pereche de dublu = doubles pair

So de dublu is not describing the pair as double in an ordinary adjective sense. It is naming the category or type of competition.

Yes, you can.

• Când se termină setul, schimbăm perechea de dublu = When the set ends, we switch the doubles pair
• Odată ce se termină setul, schimbăm perechea de dublu = Once the set ends, we switch the doubles pair

The difference is small:

• când is the most basic and neutral when
• odată ce can sound a bit more like once / as soon as

So both work, but odată ce emphasizes the moment after the set has finished.

Because the sentence begins with a subordinate time clause:

• Odată ce se termină setul = time clause
• schimbăm perechea de dublu = main clause

When this kind of clause comes first, Romanian normally separates it with a comma.

You could also reverse the order:

• Schimbăm perechea de dublu odată ce se termină setul.

That is also grammatical, though the original order sounds very natural.

It can mean either, depending on context.

In sports or organizational contexts, a schimba often means:

• to change
• to switch
• to swap
• sometimes to rotate

So here it probably means that after the set ends, the doubles pair is replaced, rotated, or switched.

The exact English word depends on the situation, but the Romanian verb itself is perfectly normal for this idea.