expression
Functions as the subject of a sentence, referring back to something previously mentioned or understood from context.
それが私の夢です。
That is my dream.
それが原因で遅刻した。
That's why I was late.
expression
well, actually ...; well, unfortunately ...
Used at the beginning of a hesitant response, often to introduce a correction, an excuse, or an unfortunate fact.
「もう終わった?」「それが、まだなんだ。」
"Is it finished?" "Well, actually, not yet."
それが、急に都合が悪くなってしまって。
Well, unfortunately, something suddenly came up.