Summon Greater Demon . oll initiative for the demon, which has its own turns. When you summon it and on each of your turns thereafter, you can issue a verbal command to it (requiring no action on your part), telling it what it must do on its next turn. If you issue no command, it spends its turn attacking any creature within reach that has attacked it.

can you then just ask the Demon for its real name and then get it ,so The demon has disadvantage on this saving throw if you say it's true name. On a failed save, the demon continues to obey you

Sounds perfectly reasonable to me. Although if I was a DM I'd rule that, if you ask it for its name, that's all it will do that round. So you can't, for example, say, "Do a leap attack at that Bugbear, and also tell me your True Name". So you have to basically waste a turn to get the name. As well, make sure that you actually speak Abyssal, because it might not answer with just its name, but might say a whole sentence like, "I am called Gleepglorp, devourer of puppies", and if you don't know Abyssal it would just sound like a bunch of nonsense with the name hidden somewhere in the middle of the sentence.

Edit: Also, let your DM know you're planning to do this ahead of time so they can come up with a better name than "Gleepglorp" on the spot like that.

Also, if you don't speak Abyssal, you may not even be able to command your demon (to tell you its name, or anything else) in the first place, if you summon one of the two examples provided which only speak Abyssal and not Common. It's a "verbal command," there's no telepathic intention-sharing involved. 

thanks had hope to see a lot more input 

Are you asking whether the demon gets Disadvantage on a saving throw to tell you its true name, on the basis that you have asked its true name? If so then no, it won't have disadvantage.

Quote from TheDungeonTomb >>

thanks had hope to see a lot more input 

 
What more input did you need?

You asked if you could just ask the demon for its true name. The answer was yes and advice given to speak Abyssal to ensure you can understand the answer. 

Your question was answered fully and completely and correctly. Was there something else you wanted to discuss as part of this?

i have seen the answers and i thank you for them. what i mean about had hope to see more input . was to see more talk about it but thanks

I have a Conjuration wizard with the Keen Mind feat. No matter the language, if it is verbal, I'll remember the name when spoken. You don't have ato speak a language to remember and speak a name though it helps.

One of my favorite tricks with Summon Lesser and Summon Great Demon is combining it with Glyph of Warding. That way I am not around when the demon is summoned.

Quote from Mergon >>

One of my favorite tricks with Summon Lesser and Summon Great Demon is combining it with Glyph of Warding. That way I am not around when the demon is summoned.

 Ooh I hadn't thought of that. I generally avoid the spells where, if you break concentration, you suddenly have another opponent on the board. But that seems like a much safer use of the spell. Although in some ways I feel like such a combination is more useful for a DM than a player, if only because players rarely get to set up traps as much as a DM does.

The spells don't truly set the Demons loose on you though, since they explicitly "pursue and attack the nearest non-demons to the best of their ability." With intentional positioning, it doesn't matter how much your demon hates you, you can 100% prevent the DM from turning them against you or the party so long as you keep at least one other non-demon between you. Including summoning defenseless animals if need be, even a Hat of Vermin or Bag of Tricks can buy you time :p

What attacks does the demon do?

Summon Greater Demons allows you to choose what demon you summon, so the attacks depend on what demon you summon.

I was not told they were gonna do it and named him John

There are four Greater Demons that can be summoned with the spell Summon Greater Demon.  The Monster Manual contains the Barlgura, a "summoner variant" of the Barlgura, there is a Reduced-Threat Hezrou from the Dead in Thay adventure and the Tanarukk from Volo's Guide to Monsters.  Only the Tanarukk lacks Telepathy.  Given that the other three can communicate telepathically, the "True Name" of a anything other than a Tanarukk might be an untranslatable concept, if you demand that it speak it's True Name out loud it may not have one you can understand in any other way than by having Telepathy yourself, or if it does have one you will need to speak Abyssal to understand it.  Demons are great liars as well.  If it has a True name that can be understood at all, and it rattles off long phrase in Abyssal, you'll have to use the whole phrase, since you have no way of knowing if only part of that is the True Name, and which part of it that is.  At least the demon gets the satisfaction of making you sound silly.

As a note to AvatarShon, I don't tend to mess with people who find old threads and make comments in them unless the last post is more than two years old.  So this is just a friendly reminder.  You appear to be very new to the forums and I thought it might be nice to give you a warning. You have committed what might be considered an offense of some kind, and you could be formally reported for it.  It's technically "posting in the wrong forum" unless you discuss Rules & Mechanics here, and I see no mention of either.  You could say that it "doesn't contribute to the discussion" since you don't say who "they" are or what "it" is.  You could say it was "Thread Necromancy" since you're making a comment in a thread from long ago.  You could say it's "Off-Topic" for the same reason why it doesn't contribute to the discussion. It could even be reported as "Other" if someone just plain doesn't like what you said for some reason or other.

It’s also worth noting that fluency in the Abyssal language is not technically required. D&D 5e lead designer Jeremy Crawford confirmed that the spell magically turns the caster’s verbal commands into comprehensible mental commands for the demon.