Interrogative (question-word) clauses

Besides their use in indirect speech, clauses beginning with question words (who, what, where, etc) can act as subjects, complements or adverbials. This structure is often rather informal (especially with how-clauses, (see here).

• Who you invite is your business.
• Where we stay doesn’t matter.
• A hot bath is what I need.
• This is how much I’ve done.
• I’m surprised at how fast she can run.
• You can eat it how you like. (very informal)

The ‘preparatory it’ structure is often used with subject clauses (see here).

• It’s your business who you invite.
• It doesn’t matter where we stay.

For who, which, etc in relative clauses (e.g. the woman who teaches me Spanish), (see here); for relative what (e.g. What she said made me angry), (see here).