Learn Hardcore Chinese: tā zhǔnbèi zài yīyuàn zhù liǎng tiān, dàjiā kěyǐ qù kàn tā. - She plans to stay in the hospital for two days; everyone can go visit her.

In this sentence, 准备 means “plan/intend to.” It can also mean “to prepare (things),” but with a verb phrase after it (like 在医院住两天) it conveys intention.

Could I use 打算, 要, or 会 instead of 准备? What’s the difference?

打算: plan/intend (very similar to 准备 here).
要: going to/need to; often sounds more definite or necessary.
会: will/likely to; a prediction rather than a plan.
All are possible, with slight nuance differences.

Why is it 在医院住 and not 住在医院? Are both correct?

Both are grammatical. The common, natural order for a short stay is 在医院 + 住 + 时长 (location before the verb, duration after). 住在医院两天 is possible, but 住在 often feels more “reside/live (at),” so it’s more typical for longer stays (e.g., 住在北京三年).

Could I just say 住院两天 instead of 在医院住两天?

Yes. 住院 means “to be hospitalized.” 她准备住院两天 is concise and natural when you mean formal hospitalization.

What grammar is 住两天 showing?

It’s a duration complement: Verb + duration (here, 住 + 两天) states how long the action/state lasts. No preposition like “for” is used.