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](https://jisho.org/search/%E5%BD%B9%E3%81%AB%E7%AB%8B%E3%81%A4)), 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。
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2021-02-23T07:30:12.590
84307
2021-02-24T00:28:55.193
39371
question
grammar adjectives
役に立ち vs 役に立つ, when to use which?
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](https://japanese.stackexchange.com/a/65953/5010).
CC BY-SA 4.0
2021-02-24T00:28:55.193
84318
2021-02-24T00:28:55.193
5010
84307
answer