《明史·志·卷一》
所以壶漏祇是用来弥补晨昏阴晦仪晷表臬的不足之处，而不是确定时辰的根本。
故壶漏特以济晨昏阴晦仪晷表臬所不及，而非定时之本。
Therefore, the water clock is only used to make up for the shortcomings of the Yin eclipse sundial in the morning and evening, not the basis for determining the time.
<s>[INST] Explain the grammatical structure of the following Classical Chinese sentence, and identify the mood and tense of verbs within it:  汉高祖既入秦，陈留守军霸上。[/INST] Grammatical structure: [...], Mood and tense of verbs: [...].</s>
Explain the grammatical structure of the following Classical Chinese sentence, and identify the mood and tense of verbs within it:  汉高祖既入秦，陈留守军霸上。
user
Grammatical structure: [...], Mood and tense of verbs: [...].
assistant