Hun er ikke bare høflig over for kunderne, men også rolig, når de bliver utålmodige.

It means not only ... but also. It links two qualities of hun:

• ikke bare høflig over for kunderne = not only polite to the customers
• men også rolig = but also calm

It is a very common Danish pattern, and it works much like the English one.

Because over for is the usual prepositional expression after høflig when you say who someone is polite toward.

So:

• høflig over for kunderne = polite toward the customers

This is a common Danish pattern:

• venlig over for nogen = kind to someone
• uhøflig over for nogen = rude to someone

Kunderne is the definite plural form, meaning the customers.

• en kunde = a customer
• kunder = customers
• kunderne = the customers

Danish often uses the definite form where English might simply say customers in a general work-context sentence. Here it refers to the customers she deals with.

Because the adjectives agree with different subjects.

• Hun er rolig: hun is singular, so the adjective stays in its basic form: rolig
• de bliver utålmodige: de is plural, so the adjective takes -e: utålmodige

This is normal adjective agreement in Danish predicate adjectives.

It refers to kunderne. So the sentence means that she stays calm when the customers become impatient.

Even though de could theoretically mean some other group in a different context, here the most natural reference is clearly kunderne.

Når means when, and it is used for something that happens regularly, typically, or whenever that situation comes up.

So når de bliver utålmodige means when they become impatient.

If you used hvis, that would mean if, which sounds more conditional or uncertain. Here the sentence suggests a recurring situation, not just a hypothetical one.

Bliver means become / get, so bliver utålmodige means become impatient.

That focuses on the change of state: the customers start out fine, then they get impatient.

If you said er utålmodige, that would mean are impatient, which describes the state itself rather than the change into it.

Because Danish often leaves out repeated words when the meaning is already clear.

So this is natural:

• Hun er ikke bare høflig over for kunderne, men også rolig ...

A fuller version would be:

• Hun er ikke bare høflig over for kunderne, men hun er også rolig ...

That is grammatical too, but less compact. Danish often prefers the shorter version.

After når, you have a subordinate clause, and Danish subordinate clauses normally keep the order subject + verb:

• de bliver

So når de bliver utålmodige is the normal subordinate-clause order.

This is different from some main clauses in Danish, where word order can change because of the verb-second rule.

Here it means calm.

Danish rolig can sometimes overlap with English quiet, depending on context, but in a sentence about dealing with impatient customers, calm is the correct and natural meaning.

Because når de bliver utålmodige is a subordinate clause, and Danish often places a comma before subordinate clauses.

You will commonly see that comma in written Danish. Depending on the comma system being followed, this kind of comma can be optional in some cases, but it is very normal and standard-looking here.