Here’s a simple statement:

Mary Anderson invented the windshield wiper in 1903.

It follows the usual pattern of English statements of SVO‏‎:

subject + verb + object

Mary Anderson + invented + the windshield wiper.

Often we ask questions‏‎ about the object of the statement. In this case we’d ask:

Q: What did Mary Anderson invent?
A: The windshield wiper.

Q: When did Mary Anderson invent the windshield wiper?
A: In 1903.

However, we can also ask questions about the subject of the statement. When we do this we simply replace the subject with a question word and add a question mark to the end:

subject + verb + object

becomes

question word + verb + object

Mary Anderson + invented + the windshield wiper.
Who + invented + the windshield wiper?

Different Question Words

Depending on the type of subject, we can use a different question word:

Professor Plum killed the Miss Scarlett in dining room with the billiard cue.
Who killed the Miss Scarlett in dining room with the billiard cue?
Professor Plum.

Winter is coming.
What is coming?
Winter is.

John’s father joined the army when he was very young.
Whose father joined the army when he was very young?
John’s.

That coat is mine.
Which coat is mine?
That coat.

Note: we cannot use when or where to ask questions about the subject.