Learn Hardcore Russian: Гость сидит у стола.

Because the preposition у requires the noun that follows to be in the genitive case. Here, стола is the genitive form of стол, which shows the relationship "at the table."

What does the preposition у mean in this context, and why is it used instead of, say, на?

У means "at" or "by" and is used to indicate proximity or nearness, suggesting that the guest is sitting next to the table. На generally indicates "on" or "upon" and wouldn’t convey the same sense of being beside something. So у correctly captures the idea of sitting by the table.

Why are there no articles like the or a in the sentence, unlike in English?

Russian does not use articles. The meaning concerning specificity or indefiniteness is derived from context rather than from words like the or a. Thus, the sentence naturally omits any articles.

Why is the verb сидит in the third person singular form?

The subject of the sentence, гость (guest), is singular. In Russian, verbs must agree with their subjects in both number and sometimes gender. Therefore, сидит is the appropriate third person singular form to match гость.

Can the word order in this sentence be rearranged without changing the meaning?

Yes, Russian word order is relatively flexible due to its case system. For example, you could say У стола сидит гость to place more emphasis on the location (the table) while still clearly indicating that the guest is the one sitting there. The cases on гость and стола ensure that the roles in the sentence remain clear regardless of the order.