Learn Swedish: Jag är trött på att vänta i telefonkön. - I am tired of waiting in the phone queue.

In English, prepositions are followed by a gerund (an "-ing" word like "waiting"). Swedish doesn't use its present participle this way. Instead, when a preposition is followed by an action, you must use att plus the infinitive verb. Therefore, "of waiting" becomes på att vänta.

Why does the sentence use trött på instead of trött av for "tired of"?

Swedish pairs the adjective trött with the preposition på to mean "fed up with" or "sick of" something. If you used trött av, it would mean physically "fatigued by" or "exhausted from" an activity, like being physically tired from a long run.

Is it always i en kö (in a queue), even if we say "on the line" in English?

Yes! In Swedish, you always stand in a queue (i en kö). Even if you are used to saying "on line" or being "on the phone" in English, Swedish strictly treats a queue as a sequence you are inside of, so i is the required preposition.

How does the word telefonkön get its ending?

It is a compound word made of telefon (phone) and kö (queue). Because kö is an en-word, it takes the definite suffix -n to mean "the phone queue". Compound words always take the gender and declension of their final noun.