Solid
Serious, Upright, Hard
い adjective
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of solid? It's solid, serious, upright, or hard. This word is often used in the figurative sense, as in a solid defence, a serious topic, or an upright person.
かたい
The reading is the same as the one you learned with the kanji.
この予想は、かなり堅いと思うよ。
I think this prediction is pretty solid.
やっぱり堅い職業の人は、書く文章も堅いね。
As I thought, people who have serious jobs write in a serious way.
レシピには、美味しくてしっとりしたクッキーって書いてあったのに、出来上がったクッキーはすっごく堅かったんだよね。
The recipe calls them delicious, moist cookies, but they turned out to be really hard.