"I eat at 1:30."
should the adjective of time be strictly fixed or it may switch places with the noun?
Um, it's grammatically an adverbial.
In Chinese, the adverbial is put before the verb in almost every cases even if the adverbial is a nasty long clause. Some adverbial, called "complement" in Chinese grammar, is put after the verb. But that's another story.
In chinese grammar, the subject must be placed before the object. This is correct because you've technically said the same thing, but turned 一点半 into the subject, making the english equivalent "at one oclock, I eat." Duolingo seems to change its mind re: when to be picky about keeping the sentence order the same or not
Would 我吃饭在一点半(Wǒ chīfàn zài yīdiǎn bàn) make sense with the extra zai? Or is it incorrect?
Is there no need for an AT character?
Also, I submitted 我吃饭一点半 (Wǒ chīfàn yīdiǎn bàn), I thought word order in mandarin was flexible, any particular reason why this was marked wrong?
First, as I've mentioned in the comment above, in Chinese the adverbial is put before the verb in almost every cases, which is different from English. So 我吃饭在一点半 is ungrammatical.
Second, the character "at" is not required in this sentence, but it is required when you want to express the location of the action (e.g. "at the end of the road").
And finally, the word order in mandarin is probably not that flexible as you may think.
Duolingo should at least tech us whst thr word means before asking us a question about it. Its like learning cosines, sines, and tangents. You cant just expect the student to solve a question using the law of sines or cosines.