Why are there two different forms of the adjective "useful"?

  1. 役に立つ
  2. 役に立ち

When I looked up on Jisho.org (here), the page would show an entry for the former even if I typed the latter in the search box.

Below are a few examples that I found (from DuoLingo.com's drills):

  1. この本はとても役に立ちます。
  2. この本は日本の歴史を学ぶのに役に立つ。
  3. 役に立つ文章はどこですか。
  4. 彼は役に立ちません。
  • 4
    Does Duolingo not teach the difference between verbs in plain form and polite form?
    – Leebo
    Feb 23 at 8:29
  • I am taking another Japanese course (but drilling on DuoLingo), and I guess this topic has not yet been taught. Thanks for the pointer. I was guessing the question was a grammar topic but I didn't know what it is and how to read up on it on the Internet ... The course I took is みんなの日本語  I am on Chapter 11 of the first book (of 12 chapters)
    – leeyuiwah
    Feb 23 at 10:08

First, note that 役に立つ is not an adjective but a verb-based set phrase (do you remember ます only follows a verb?). If you want an adjective that directly corresponds to the adjective 'useful', use 便利な (na-adjective) or 有用な (na-adjective) instead.

立ち is one of the many forms of the verb 立つ. I don't know how your textbook calls the former, but it's commonly called masu-stem, verb stem, i-form, pre-masu-form, continuative form, etc. If you have completed the first few chapters of your textbook, I believe you have already learned them. If you want to review this form, please read this answer.

  • Thank you very much!
    – leeyuiwah
    Feb 24 at 7:25
  • I appreciate the answer. However, the suggested link does not actually talk about 立つ, but I found this page (which apparently was auto-generated) talks about it japaneseverbconjugator.com/… I now realized that this topic is at a level that I have not learned, but I think it is good to look ahead a little bit in my learning. Cheers!
    – leeyuiwah
    Feb 25 at 17:14

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