じゃあ details
Examples
well then; in that case — proposes what follows from what the other person just said
「今日はちょっと無理なんだ。」「じゃあ、明日の夕方は？」
"kyou wa chotto muri na n da." "jaa, ashita no yuugata wa?"
"I can't make it today." "Well then, how about tomorrow evening?" casual
資料は全員に行き渡ったようですね。では、本日の議題に入ります。
shiryou wa zen'in ni ikiwatatta you desu ne. dewa, honjitsu no gidai ni hairimasu.
It looks like everyone has the handouts. In that case, I'll get started on today's agenda. neutral
それでは、ただいまより開会の辞を申し上げます。
soredewa, tadaima yori kaikai no ji o moushiagemasu.
With that, I will now deliver the opening address. formal
well then — closes the exchange and lets both sides leave
じゃ、また来週ね。
ja, mata raishuu ne.
Right, see you next week. casual
Formation
じゃあ、clauseattaches to plain clause
Notes
ではありません (学生ではない), and mid-sentence it is the particle で
plus は (ここでは静かに). The connective only stands where a sentence could start.
Related patterns
Similar
なら needs its condition spelled out inside the sentence (雨なら中止); じゃあ takes the other person's whole last turn as that condition and can only stand at the head of a new sentence — they overlap exactly when the condition is what was just said.
Both open a new sentence at a turn in the conversation, but ところで drops the previous topic and needs no link to it, while じゃあ is licensed BY the last turn and draws its next move from it — じゃあ with nothing to build on reads as a non sequitur.
Lookalikes
One は apart from それでは — the formal end of the じゃあ entry — and both react to the last turn, but それで points backwards at a result that already followed (「それで？」asks for the rest of the story), while それでは／じゃあ points forwards and announces the next move.