Learn Swedish: Jag vill att du tömmer lufttuben. - I want you to empty the air tank.

English uses an object plus an infinitive ("want you to empty"), but Swedish doesn't allow this structure with verbs expressing desire or will, like vilja. When the person wanting is different from the person doing the action (a subject shift), you must use a subordinate att-clause. Literally, you are saying "I want that you empty".

Why is the verb tömmer in the present tense here instead of the infinitive?

Because att du tömmer is a full subordinate clause, it requires a conjugated verb rather than an infinitive. Since the main clause is in the present tense (Jag vill), the subordinate clause also uses the present tense (tömmer) to express the desired action.

If I wanted to empty the tank myself, would I still use an att-clause?

No. If the subject remains the same, you just use the infinitive directly after the verb. "I want to empty the air tank" would be Jag vill tömma lufttuben. You only force the att-clause when a new subject is introduced for the second verb.

How is the word lufttuben constructed?

It is a compound of luft (air) and tuben (the tube or tank, from en tub). They join directly together without a connecting "s", which is very common when the first part of the compound is a base noun like luft.