

姉は足が細いです。
この道は細いので、気をつけてください。
もう少し細いペンはありますか。
"ほそい(細い)" is in the lesson "Adjectives 3" (Level 7), along with 13 other useful words. LinDuo has 180 topic lessons like this one, 2,379 words in total. Every word has an illustration, a transcription and pronunciation in a female and a male voice. Just 10–15 minutes a day is enough to learn them all. Free, and works offline.