Learn Danish: Jeg ved ikke hvem der planlagde indbruddet. - I do not know who planned the burglary.

This is the core rule for indirect questions! When a question word like hvem (who) or hvad (what) is the subject of a subordinate clause, Danish requires you to insert the dummy pronoun der. Without der, the sentence would be grammatically incomplete in Danish, even though it translates directly to English without it.

Would I still use der if the sentence was about who someone else saw?

No, you would simply say Jeg ved ikke hvem han så (I do not know who he saw). In that sentence, han (he) is the subject. You only insert der when the question word itself is doing the action. Since hvem is the one doing the planning in our main sentence, we must add der.

Why does indbruddet have a double d?

The root word is et indbrud (a burglary). When adding the definite suffix -et to make it mean the burglary, Danish spelling rules require you to double the final consonant to keep the preceding vowel sound short. Therefore, indbrud becomes indbruddet.

How does the verb planlagde get its past tense form?