The particle مگر magar is used in the interrogative sentences in the following situations:

When the speaker expects an opposite reply, even if the answer is indeed agreeable:

Possible answers:

بله٬ آمده‌اند
bale, āmade-and
“Yes, they are.”

نه٬ نیامده‌اند
na, nayāmade-and
“No, they are not.”

Possible replies:

نه٬ نرفتم
na, naraftam
“No, I did not go.”

چرا٬ رفتم و برگشتم
cerā, raftam va bargaštam
“Yes I did go and I (just) returned.”

مگر نگفتی دیگر این کار را نمی‌کنی؟
magar nagofti digar in kār rā nami koni?
“Didn’t you say you wouldn’t do this again?”

مگر شما هم می‌آیید؟
magar shomā ham mi āyid?
“Are you coming, too?”

Or:

مگر نمی‌آیید؟
magar nami āyid?
“Aren’t you coming?” “Aren’t you going to join me/us?”

which means the speaker expected the listener/receiver to go or join him/her/them.

It also occurs as the segment مگرنه؟ magar na? when the speaker induces agreement to a statement he or she makes (negative or positive), more like “Isn’t it true?”, or “Am I not right?”:

این خراب است٬ مگرنه؟
in xarāb ast, magar na?
“This is broken, isn’t it?”

این را می‌خواهید٬ مگرنه؟
in rā mi xāhid, magar na?
With the stress on in = “You want this (one), don’t you?”
With the stress on mi = “You do want this, don’t you?”

حرفم را‌می‌فهمی٬ مگرنه؟
harfam rā mi fahmi, magar na?
“You do understand (me), don’t you?”
“We do understand each other, don’t we?”

حرفم را‌فهمیدید٬ مگرنه؟
harfam rā fahmidid, magar na?
“You did understand, didn’t you?”
“You did get my drift, didn’t you?”
“You do understand me, don’t you?”

حرفم را نفهمیدی٬ مگرنه؟
harfam rā nafahmidi, magar na?
“You didn’t understand me, did you?”
“You did not understand me, am I wrong?”
“Correct me if I’m wrong, you did not understand me, did you?”

In addition, مگر equates to the English conjunctions “unless” and “unless otherwise…”:

این را به او می‌دهم٬ مگر این که خودش نخواهد
in rā be u mi daham; magar in ke xodaš naxāhad
“I’ll give this to him/her, unless he/she himself/herself doesn’t want it.”

این را به او نمی‌دهم٬ مگر این که خودش بخواهد
in rā be u nami daham; magar in ke xodaš bexāhad
“I shall not give this to him/her, unless he/she himself/herself wants it.”

مگر نبینمش٬ می‌کشمش
magar nabinamaš, mi košamaš
“Unless I don’t see him/her, (or else, otherwise) I shall kill him/her!”

Sometimes the phrase وگرنه vagar na (va agar na ‘otherwise, or else’), intervenes, as follows:

مگر نبینمش٬ وگرنه می‌کشمش
magar nabinamaš; vagar na mi košamaš
“Unless I don’t see him/her, (or else, otherwise) I shall kill him/her!”

Or even که ke intervenes:

مگر که نبینمش٬ وگرنه می‌کشمش
magar ke nabinamaš, vagar na mi košamaš
“Unless I don’t see him/her, (or else, otherwise) I shall kill him/her!”

Sometimes magar expresses utter curiosity or surprise:

مگر او کیست؟
magar u kist?
“Who is he/she? Who in the world is he/she?”

مگر تو کی هستی؟
magar to ki hasti?
“Who the hell do you think you are?”

مگر چه گفتم؟
magar ce goftam?
“What (in the world) did I say (that made you so angry)?”

مگر نیامده‌اند؟
magar nayāmade-and?
“Don’t tell me they are not here yet!”

مگر چه کار کردم که از من کناره می‌گیری؟
magar cekār kardam, ke az man kenāre migiri?
“Tell me, what did I do that you keep away from me?”

مگر چه کار کرده‌ام که از من کناره می‌گیری؟
magar cekār karde am, ke az man kenāre migiri?
“Tell me, what have I done that you keep away from me?”

Also the colloquial:

مگه چه‌کار/چیکار کردم که از من کناره می‌گیری؟
mage cekār/cikār karda|m, ke az man kenāre migiri?
“Tell me, what have I done that you keep away from me?”