Learn Hardcore Japanese: ryokou no zyunbi ga owarimasita. - I have finished preparing for the trip.

旅行の準備 literally means “preparations for a trip.” The particle の links 旅行 (“trip”) to 準備 (“preparation”), showing a relationship much like “’s” or “of” in English.

Why is the particle が used before 終わりました instead of は?

The particle が marks 準備 as the subject of the verb 終わりました and simply states that it finished. Using は would make 準備 the topic and could imply contrast (e.g. “As for the preparations, they finished, but…”).

Why is 終わりました in the past tense and polite form?

Past tense (終わりました, from 終わる) indicates the action is completed.
The -ました ending makes it polite, suitable for formal or neutral contexts.
In casual speech you might say 終わった instead.

Could I say 旅行の準備を終えました instead? What’s the difference?

Yes. ～を終える is a transitive verb meaning “to finish something.”

旅行の準備を終えました focuses on the person completing the action.
旅行の準備が終わりました focuses on the preparation itself being finished.
Both are correct and often interchangeable.

Why is there no explicit subject like “I” in the sentence?

Japanese commonly omits the subject when it’s clear from context. Here, it’s understood that I (or we) did the preparing, so 私は or 私たちは isn’t necessary.

Can 準備 be used with other nouns in the same way?

Absolutely. You can say:

仕事の準備が終わった (“Work preparations are done”)
料理の準備が終わった (“Cooking prep is done”)
Basically [noun]の準備が終わる works whenever you finish preparing something.

How would I say “I am preparing for the trip” to emphasize an ongoing action?

Use the ～ている form:

旅行の準備をしています – “I am doing the trip preparations.”
Or to stress progress:
旅行の準備を進めています – “I am moving along with the trip prep.”