"It is a desk."
So you'll learn it. When I don't know what I am reading, I click on the Japanese and learn it.
In my case, all the other options had a number in them, so it wasn't hard, but i get your point.
Tap the word and the translation will appear. It's just Duolingo's method of instruction.
It doesn't, if you have to choose the hiragana underneath the english sentence.
I believe 机 (つくえ) is a surface you do work at, while テーブル is a surface you eat at. I could be wrong though, since I'm not a native speaker.
One question. 三つ＝みっつ，while七つ＝ななつ so when should I add 'っ'，I am getting confused...
The small っ is added for 三つ because one of the readings of 三 is みつ (as in the company 三菱 Mitsubishi). However, for ease of pronunciation, when the kanji is used with the general counter (big) つ, it becomes みっつ. I don't know why 五つ (いつつ) doesn't do this though...
Why can it say "It is 'a' desk" like meaning one (it also says one when you click on 'a') But when you try to add一つ it counts it as wrong?
It depends how you added it. (And also that it would be more specific than the original sentence.)
Arguably, "It is a desk" and "It is one desk" mean roughly the same thing, 一つのつくえです could be acceptable when you're translating from English to Japanese, but because Japanese doesn't differentiate between plural and singular, when translating つくえです to English, it can mean "It is a desk" or "they are desks". Perhaps it's an oversight on the part of the course developers to disallow 一つ because it's not a symmetric translation.
However, つくえの一つです means "It is one of the desks", つくえが一つです means "There is one desk", 一つがつくえです means "One (of the things) is a desk", etc. No other combinations I can think of work for it to mean "It is one desk".
Is there a difference between 机はです and 机です. To me, both seem to be the same since 'desk' is the subject in both.
机はです is grammatically incorrect*. です is a special kind of verb, especially in such short sentences.
If you think of the general SVO sentence "A is B", where A is the subject (in English) and B is the object (in English), you would have to write it as AはBです in Japanese. If it's obvious from the context what A (the topic) is, you can leave it (and は) out, but if you remove B (the object), you have an incomplete sentence - "A is ..." what?
*Side note: you can leave out B too if it's very obvious from the context, but it's uncommon and this basically only happens if someone is asking you to reconfirm the topic and/or you want to prevent repeated misunderstanding of the topic.
I accidentally said つくえ and it was marked as correct. Is it actually correct?
As in, with no です? In some circumstances, sure, but you have to be careful because it can sound rather curt or rude.
If you meant, instead of テーブル, the suggested answer for this question is つくえです, so yes, it's actually correct.