Learn Swedish: Han lär barnen att simma. - He teaches the children to swim.

The verb att lära is tricky because it can mean both "to teach" and "to learn"! However, when it means "to learn", it is reflexive (used with a pronoun like sig). Since there is no reflexive pronoun here, and he is doing the action to barnen, it means "teaches". If he were learning to swim himself, it would be Han lär sig att simma.

Why is "the children" barnen? Doesn't the -en ending usually mean it's singular?

For most en-words, adding -en makes them singular definite (like bilen for "the car"). But barn is an ett-word, and its plural form doesn't change (ett barn, flera barn). For ett-words that end in a consonant, the definite plural ("the [things]") is formed by adding -en. Therefore, barnen translates to "the children", not "the child" (which would be barnet).

Why do we need the word att before simma?

In Swedish, att is the infinitive marker, just like the word "to" in the English phrase "to swim". You generally use it when one verb follows another, unless the first verb is a modal verb (like kan, vill, or måste). Since lär is not a modal verb, we need att before the infinitive verb simma.