Weak
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 weak? It's also weak.
よわい
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you: You just got beaten up because you're weak. Then the bully asks you: "Yo! Why (よわ) you so weak?"
父は体が弱いんです。
My father is physically weak.
手と足の力が弱くても、バドミントン上手になれますよ。
Even if the strength of your hands and feet are weak, you can get good at badminton.
弱い火の光でも、辺りが少し明るくなりました。
Even with a small light of fire, it has become a little brighter around.