For ungrammatical sentence like "*我吃早飯在八點", it is firstly a kind of negative transfer from their mother tongue English that causes the error. Since in English the normal form of this sentence is "I get up at 8 o’clock." The preposition "at" in English is obligatory here to signal "time", but in Chinese, "在" cannot exist before temporal adverbial except for the ‘在…的時候’ structure.
Secondly, time adverbs are commonly placed before the verb in Chinese, so here, by placing "8點" before the verb, we can get "八點我吃早飯" and "我八點吃早飯", and both are acceptable. Compare these two sentences, the default form is the second one, and by moving the time adverb to the front, we topicalized the time "八點", and thus the time here becomes more important in pragmatic level.
Statistics shows that, time adverbials can normally appear at the beginning, in the middle and at the end of the sentence in English, and to place them at the end is the most common way.
Now I feel it’s very easy to get inside.
Career women now shop at couture.
"I removed them all," the doctor declared, "we’ll close now."
While in Chinese they usually precede the verb that they modify and are placed before or after the sentence subject, with different pragmatic focuses, while the possibility of using which one of them is relatively equal.
現在我們可以看到許多華人在美國成就卓然。
你現在可以走了。
To place the time adverbs at the sentence final position is rare, and thus it is the more marked way for temporal adverbials in Chinese for some specific meanings or in special structure.
真想大哭一場，趁著眼睛還有淚水的時候。(行鷹《生命之水》)
