翻译：The shape of the table was very important.
Well, if you got this sentence in past tense section, I think it is better to follow the rules.
The word Be is a verb for sure, but it's a copula, or a link verb, in Chinese we call it 系动词. The tricky part of a Chinese sentence is you cannot really understand its tense without a proper 助词 (Chinese particles), such as 了, 的, 着, 吧, etc. In this particular sentence, 是 can be translated into Is or Was both ways. Let's try to translate this sentence into English: 我就这样，你能把我怎么样？ What tense would you use?
Moreover, there is no concept of "tense" in Chinese grammar. Tense is a concept of using verbs in English and some other languages, it is translated as 时态 in Chinese. The verbs in Chinese never change its form (state). If we translate 时态 in English, it means Time and State (or Form, or Condition) . But there is concept of Time and State, Form or Condition in Chinese. 虚词 (form word, empty word, or grammatically structural word) is often used to emphasize the "Time and Form" of an action (verb), 虚词 includes grammatical particles, conjunctions, copulas, prepositions, interjections, even adverbs and more, such as 在，也，着，过，之，吧，啊，的, etc. We cannot discuss about "Tense" in Chinese, but we can discuss about how to state "Tense" in Chinese. (by using grammatically structural words 虚词).
You have a great command in Chinese, I have to say : ). But, if......duolingo accept(s) present tense in past tense section, I can't think of any good done to the English learning. They might have to add something like this
这桌子的形状是很重要的(past tense)/(present tense).
BTW, are you a teacher or Chinese literature student?
Thank you for your kind comments! I'm just a guy who's interested in learning English and Chinese. I got quite confused when I was trying to compose any Chinese classical poems and couplets, though I'm a native Chinese speaker. I'm still trying to learn Chinese now. I've learned English by myself all these years and I've noticed that finding a better way is more important to learn a language. I happened to watch a video clip last night about the founder of this website, so I came to see how it worked. I've also commented about other sentences, too, and I would like to hear your opinions: