How the question changes depending on what you're asking about

In English, the structure of a question depends on whether you're asking about the subject (the person or thing doing the action) or the object (the person or thing receiving the action). This distinction matters because it changes word order — and one type of question is much simpler than learners expect.

Object questions are the ones you already know well. You're asking about the thing that receives the action, so you need an auxiliary verb (do/does/did, is/are, has/have, etc.) and inverted word order:

✅ Who did Sarah call? (Sarah called someone — you're asking about the object.)

Subject questions are different. You're asking about the person or thing that performs the action, so there's no inversion and no added auxiliary. The question word simply replaces the subject and the sentence keeps its normal order:

✅ Who called Sarah? (Someone called Sarah — you're asking about the subject.)

Notice the difference: Who called Sarah? has no "did." The verb stays in its original tense form, just as it would in a statement (Tom called Sarah → Who called Sarah?).

Telling them apart in practice

Here's a reliable test. Start with a statement and decide which part is unknown:

Statement: Something broke the window. You don't know the subject → What broke the window? (subject question — no auxiliary needed)

Statement: The children broke something. You don't know the object → What did the children break? (object question — auxiliary "did" + base form)

One more pair, this time with "who":

✅ Who teaches this class? (subject question — someone teaches it) ✅ Who does this class teach? (object question — the class teaches someone, perhaps in a peer-teaching context)

Both sentences are grammatically correct, but they mean completely different things.

The mistake almost everyone makes

The single most common error at B1–B2 level is adding "do/does/did" to a subject question:

❌ Who did call you last night? ✅ Who called you last night?

Learners have drilled auxiliary inversion so thoroughly that it feels wrong to leave it out. But in a subject question, the question word is the subject, so the sentence already has normal subject-verb order. Adding an auxiliary creates either an error or an unintended emphatic meaning ("Who did call?" sounds like you're insisting someone must have called).

The same thing happens with "what" and "which":

❌ What does make you happy? ✅ What makes you happy?

❌ Which bus does go to the airport? ✅ Which bus goes to the airport?

Notice that the verb carries the third-person -s (makes, goes) because the question word is acting as the subject.

Edge cases to watch for

With "be" as the main verb, both subject and object questions use inversion, so the distinction is less visible:

Who is your teacher? (asking about the subject or complement — either way, "is" inverts naturally)

This means the subject/object rule mainly affects questions with action verbs.

With "who" vs. "whom": Technically, "whom" signals an object question (Whom did you invite?), while "who" can be either. In modern English, most people use "who" for both, so you can't rely on the pronoun alone to tell the question type — look at the word order instead.

Present simple and past simple are where the trap is sharpest, because those are the tenses where "do/does/did" appears only in object questions. In tenses that already have a visible auxiliary (is running, has eaten, will go), the difference is subtler because the auxiliary is there regardless:

Who is running the meeting? (subject) vs. Who are you meeting? (object)

Both have a form of "be," but the subject and verb still swap positions in the object question.

Keep it simple: if the answer to your question would be the subject of the sentence, don't add an extra auxiliary. Let the question word do the subject's job and leave the verb exactly where it would sit in a statement.