Sad
い adjective
This is a single kanji with しい on the end, so you know it's an adjective. What's the adjective form of sad? It's sad, sadly.
You learned this reading with 悲しむ, hooray!
悲しいけど、しょうがないよ。
It's sad, but it can't be helped.
それは悲しすぎるね。
That's too sad.
モモちゃんが悲しい気持ちの時は、お父さんもお母さんも悲しい気持ちになってしまうのよ。
When you feel sad, Momo, neither your dad nor myself are happy.