How the word "かせん（下線）" is translated into English
大切なところに下線を引いておきました。
申込書の下線の部分に名前を記入してください。
赤い下線がある箇所は修正が必要ですか。
"かせん（下線）" is in the lesson "Application" (Level 40), along with 12 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.
