Learn Dutch: De docent is het document aan het typen. - The teacher is typing the document.

They are doing two completely different jobs! The first het is simply the word "the" for the neuter noun document (het document = the document). The second het is a mandatory part of the fixed grammar phrase aan het, which acts like the English "-ing" to show an ongoing action.

Why is het document placed in the middle of the sentence instead of at the end?

In English, you keep the verb together: "is typing the document". But in Dutch, this verb structure splits in two. The main conjugated verb (is) takes its normal spot near the beginning, while aan het typen gets pushed all the way to the end of the clause. Any objects, like het document, get sandwiched in between them.

What does aan het literally mean here?

It literally translates to "on the" or "at the". If you translate this sentence word-for-word, it says "The teacher is the document at the typing." This comes from an older way of speaking (similar to the old English "he is a-working"), but in modern Dutch, it is just the standard formula used to express that an action is happening right now.

I learned the word leraar for teacher. What is the difference between a docent and a leraar?