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