There are two verbs in Chinese that both mean "to be". Basically - 是 for 是noun and 很 for 很 adjective. 我是學生 I am a student 我是英國人 I'm a British person 這是桌子 This is a table 他是我的朋友 He is my friend 她是我的媽媽 She is my mother 他很胖 He's fat 她很瘦 She's thin 哎呀，這個很麻煩 - Ah damn, this is troublesome 你很高啊 - You're tall! 她非常漂亮啊！我很嫉妒！She's really beautiful! I'm jealous! 是 can be used with adjectives when it's combined with 的 這是紅色的 - This is red (literally - this is a red one) 這是黑色的 This is black 這是白顔色的 This is white 他是很胖的 He is a fat (He is a fat one) However, it's most commonly used to clarify details 我是2015年去中國的 It was in 2015 that I went to China 我跟朋友去緬甸的 - I went to Myanmar with my friend Laurence

桌子是圆的，that is right.

Yes. A 是 must come before that kind of adjective. And a 的 must come at the end of the sentence, as you may know. The reason for that may be that we usually say noun + 很/非常 + non-absolute adjective rather than noun + 是 + non-absolute adjective + 的 while absolute adjectives cannot be modified by 很 or 非常. So we have to say noun + 是+absolute adjective

中国老一辈语言学家曾经提出：尽量少用”是“。可见”是“在汉语里并非象我们现在理解的多么重要的东西。现在更多的用”是“表达的方式来自西方语言的翻译。大家有兴起可以去看一下明清小说，看看那时人们的话里有多少”是“。