It's just formal French, you'll find it mostly in books or articles.
It's to complete the word "ce/cet/cette", to mean this exact hour, not any other one. "-là" can also be used the same way.
But these days it doesn't add any emphasis anymore. You could take off the "-ci" or "-là" it would not change the sentence at all.
Note that "-là" is often used in oral French.
Sorry, I don't fully agree with the fact that the need for emphasis would have disappeared. Imagine when your son comes back home late, you ask him: "c'est à cette heure-ci que tu rentres ?"
There are also expressions like "ces temps-ci" meaning "nowadays" or "ces jours-ci" meaning "lately".
For your first sentence, "C'est à cette heure que tu rentres ?" would make the same effect, only we're more used to see it with the "-ci" or "-là".
I don't know, maybe some people still see an emphasis, but I personally don't feel it, neither in oral nor written French. It feels more like a "legacy" from the old French, but doesn't really add anything to the language anymore.
As for "ces temps-ci", "ces jours-ci" etc... they are special expressions, which can't be used without the "-ci" anyway. Those are different.
Ha, ha! "Il vient toujours à sept heure-ci" is accepted. LOL! I thought the same thing as you did at first so I typed it that way. I kind of realized that it was probably cette heure-ci, so I changed the "heures six" to heure-ci, but forgot to change the "sept" to "cette" but it still was marked as correct. :D
The only way to report it is to do the exercise again. If you remember the lesson, just do the lesson until you find the correct exercise, and report it. Don't worry if it's annoying for you, other people will be able to report it sooner or later.
Unlike @Sitesurf, I think it needs to be reported, because there shouldn't be any liaison pronounced for this sentence.
Well yes, I know, and that's why I disagree. The liaison is not an option in my opinion. Not with "toujours", it's a forbidden liaison.
I've found a few links concerning this :
About.com as usual has a very helpful explenation for the suffic -ci as it calls it. It certainly helped me to understand it clearly.
Yes, unless context is clear enough:
- Even though I told him many times that the time was not appropriate, he still comes at this time = Bien que je lui aie dit maintes fois que ce n'était pas le bon moment, il vient encore/toujours à cette heure-ci
- Whatever happens, he always comes at this time = Quoi qu'il arrive, il vient toujours at cette heure-ci.
"Time" can have various translations, including "heure" and "fois".
- It is time to do it = Il est temps de faire ça OR C'est l'heure de faire ça
- At this time = à cette heure-ci
- At that time = à cette heure-là
- This time = cette fois-ci
- That time = cette fois-là
And "temps" is used to translate other words than "time":
- Ces temps-ci = these days
- En ce temps-là = in those days
Most of those are idiomatic in both languages, so you have to learn them as they come.