Both words have the same meaning and function (conjunction and preposition), and are largely interchangeable. "Till" is more likely to be heard in speech: Just wait till we get home!. "Until" is more usual at the beginning of a sentence: Until we actually get on the train we won't know our destination. The spellings
'till reflect the commonly held belief that till is a shortened form of until, but till is in fact the older form. Until appears to have been formed by the addition of Old Norse
und: "as far as", several hundred years after the date of the first records for till.
Generally, you'll say "Bonsoir!" when entering a room / meeting people / at the start of a conversation or event / etc., i.e. the equivalent of "Hello / Bonjour" after sunset.
Whereas "bonne soirée" is something you wish when you leave people or at the end of a conversation, event,... The equivalent of "Have a nice evening!"
The one-word stuff will be as a welcome "Bonjour (morning and afternoon) / Bonsoir (evening and night)", whereas the two-word stuff will be when leaving / "Bonne journée (start of the day)! Bon après-midi (after noon)! Bonne soirée (end of the day/evening)! Bonne nuit (at night)".
First, "voit tu plus tard" makes no grammatical sense, even as a word for word translation from English. In "See you later", I suppose you do realize that "you" is not the subject : "See you later" actually implies "[I (will)] see you later". "You" is here the object.
So it couldn't be "tu" anyway as "tu" is only the French subject for "you" ; the object form is "te". Also, as "I" is the subject, i.e. "je" in French, the verb "see" should be "vois" in French (I see = je vois)
Besides, we generally don't "skip" pronouns in French, like in English such as "see you later" or phrases like "gotcha / got you !".
From there, "see you later" in a proper direct translation in French makes :
- "Je te vois plus tard"
That's a 100% correct sentence, and can be used in this very context. It's just that those types of sentences are almost set in stone, they're automatic expressions with each language having its "preference" in terms of formulation. "À plus tard" is more common than "je te vois plus tard" (unless you want to insist and be less "casual" about it).
Yet in fact, both phrases in both languages are interchangeable. In "À plus tard", the "à" is in fact an equivalent for "until / till" in English ; let's see with "next time" instead of "later" (sounds better) :
- "à la prochaine fois !" = "till next time !", literally. But it's more formal in English, whereas in French it's more common and informal.
- "je te vois la prochaine fois / on se voit la prochaine fois !" = "[I] see you next time / we see each other next time" (again, this is literal and just for you to understand).
So no it does not happen as a rule, it's mainly a question of usage in different languages.
You're right, "ai" as a diphtong (mind the spelling) is pronounced [é] or actually [è], but the following "-n" changes its pronunciation.
First, "ai" is most commonly an [è] sound, examples :
la haine (hatred) = la [ènn] (note that here the "-n-" has no influence on the diphtong because of the final "-e", so you pronounce the [n] properly)
un balai (a broom) = un [balè], exactly like "un ballet", a ballet !
ouais ! (yeah !) = [wè] (the "s" is mute, as most final "s" in French)
du lait (milk) = [lè]
Sometimes, usage makes the pronunciation tend towards the [é] sound (as in the word "fiancé"), example :
- j'ai (I have) = [jé]
But the unpronounced "-n" changes the sound of the diphtong "ai", so that "-ain" actually sounds like "-in" or "un", the word for "a" or "one", i.e. "1" ; that's why in smartphone text language, we sometimes write those words using the number "1" to make them shorter : "demain" can thus be written "dem1" (a bit like "l8" to mean "late").
All the following words sound the same :
une main (a hand) = une [m1]
du pain (bread) = du [p1]
un pin (a pine) = un [p1]
un frein (a brake) = un [fr1]
un joint (a joint) = un [jw1]
juin (June) = [ju1]