If you mention this, you'll touch him on a tender spot.

你要是提及这事，会触及他的痛处。

youdao

Once you find a tender spot, you can focus and press there, but not too hard.

一旦找到那个柔弱的部位你就可以集中这点施压稍用力但别太重。

youdao

You've touched me on a tender spot, ie mentioned sth I find painful or unpleasant.

你触到了我的痛处（提及使我痛苦或不快的事）。

youdao

For many who have who always celebrated holidays with warmth and abandon, there's a raw and tender spot where a departed family member used to be.

CNN: A depressive's guide to Christmas