Why is there no 的 in the question 你是第一次去北京吗 (is this your first time going to Beijing)? This is from our Chinese book (entitled 会通汉语 读写1, and available from 京东):


莉莉：这是我在北京拍的照片。
何大华：你是第一次去北京吗？
Lìlì: zhè shì wǒ zài Běijīng pāi de zhàopiàn.
Hé Dàhuá: nǐ shì dìyīcì qù Běijīng ma?

(As I understand, Lili (莉莉) says these are the photos she took in Beijing, and He Dahua (何大华) asks if this was her first time in Beijing.)
I'm confused about He Dahua's grammar: he seems to be using a 你是[something]的 construction, but in this case the 的 is omitted.  It reads like Are you &quot;the first time going to Beijing&quot;? which doesn't make sense to me (she's a human, not a trip to Beijing).
I feel 你是第一次去北京的吗？ is better, but maybe I'm wrong.
Question: Why is there no 的 in the question 你是第一次去北京吗?

The question What&#39;s the grammatical structure of 这是你第一次来中国吗？ is similar, and asks about:

这是你第一次来中国吗？

This is slightly (but importantly) different since the order in the linked question is 是你第一次..., while in this question the order is 你是第一次...:

你是第一次去北京吗？

I wouldn't be confused if the question instead were 是你第一次去北京吗？
The structures are different

[1. 你] [2. 是第一次去北京] [3. 吗]?
[1. you (subject)] [2. are going to Beijing for the first time (verb clause)] [3. final particle indicates question form]

The verb clause &quot;是第一次去北京&quot; describes what you 'do'- you go to Beijing for the first time
~

[1.你] [2. 是第一次去北京的] [3.吗]?
[1.You (subject)] [2. 是第一次去北京的 (adjectival clause)] [3. final particle indicates question form]

The adjectival clause '是第一次去北京的' describes what 'you' are - 'you are the person who goes to Beijing for the first time'
Other example:
我不会屈服 I will not yield (describe what I do)
我是不会屈服的 I am a person who will not yield (describe who I am)