对此(duì cǐ) - regarding this, about this in Mandarin

对此 (duì cǐ) points back at whatever was just said and frames the next clause as a reaction to it. It is the written equivalent of "on this", "about this" or "in response to this", and it very often opens the sentence.

Because 此 is the literary word for this, the whole phrase carries a formal register. You meet it in news reports, official statements and business email far more than in conversation: 对此，公司表示遗憾. It also sits comfortably mid-sentence after the subject, as in 我对此没有意见.

The trap is that 此 is not 这, and swapping them does not work. 对这 is not a phrase; the spoken equivalent is 对这件事 or 对这个 with a noun after it. Learn 对此 as a fixed unit that already contains its object, and never put another noun behind it.