Jenny:The frame is square.
This sentence is correct~! ^^

Jenny: The signs is triangle.
Correct: The signs are triangle.
Explanation: We use ARE, if the subjects are plural (two or more), and we use IS if the subject is singular.

Jenny: The egg is oval.
This sentence is correct~! ^^

Jenny: The book is rectangle.
This sentence is correct~! ^^
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Polished:
1. The clock is circle.
2.The frame is square.
3. The signs are triangle.
4. The egg is oval.
5. The book is rectangle.
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Teacher's Comment:
You have a great potential toward achieving it! I love the way you exhibit your interest and enthusiasm for the things we did. You are gaining more self confidence, keep it up! You are on the right track to achieving great English skills if you keep your mind on concentrating and learning more.

-Teacher Andrea~ ^^