Thin
Slender
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 thin? It's also thin, though it can be slender as well.
If your garden is too thin, just get out the hose (ほそ)! How about if your hair is thin? Point the hose at your head! Hoses can cure any thin issues you may have.
もう少し細いペンはありますか？
Do you have a pen that is a little finer?
この糸はちょっと細すぎますね。
I think this thread is a bit too thin.
ピアニストみたいに細くて長い指だね。
Your fingers are slender and long like a pianist's.