Learn Hardcore Chinese: tā shēngbìng le liǎng tiān, xiànzài shēntǐ bǐ yǐqián hǎo duō le. - She was sick for two days; now her health is much better than before.

The 了 after 生病 in 她生病了两天 is the perfective/aspect 了. It marks a completed event and lets you attach a duration: “was sick for two days (and that episode is over).”
The 了 at the end of 好多了 is the sentence‑final/change‑of‑state 了. It signals a new situation now: “is much better now.”

Both can appear in one sentence because they do different jobs: one closes off the past event; the other points out the new current state.

What’s the difference between 生病了两天 and 生病两天了?

生病了两天 = “was sick for two days.” It presents the sickness as a completed episode.
生病两天了 = “has been sick for two days (up to now).” It suggests the sickness is/was ongoing up to the reference time. In your sentence, the first one fits because the next clause says she’s much better now.

Is 生病 a separable verb? Can I also say 生了两天病 or 病了两天?

Yes. 生病 is a separable verb (离合词). Common variants:

她生病了两天 (very common in modern usage)
她生了两天病 (shows the separation explicitly; perfectly fine)
她病了两天 (also very natural and colloquial) All three are acceptable here and mean the same thing.

Why is it 两天 and not 二天?

Use 两 before measure words (days, hours, people, etc.): 两天、两个小时、两个人. Use 二 mainly for numbers in counting, phone numbers, addresses, math, and dates (e.g., 二楼, 二号).

What does 多 do in 好多了? Is it the same as “many”?

Here 多 is an intensifier meaning “much/a lot,” not “many.” 好多了 = “much better (now).” You can express the same idea in a few ways:

比以前好得多 (neutral/clear)
比以前好很多 (neutral)
比以前好多了 (very natural in speech; 了 adds “now” feeling)

Can I omit the final 了 and say 现在身体比以前好多?

You sometimes hear 比以前好多, but the most natural update-style statement is 比以前好多了, because sentence‑final 了 highlights the change in the current state. Without 了, it sounds more like a general comparison, not an “it’s improved now” update.

Could I use 更 instead of 比? For example, 现在身体更好了?

更 = “even more/–er (than before/than expected).” You can say 现在身体更好了 if the context already implies what you’re comparing to.
If you want to say it explicitly, use both: 现在身体比以前更好 (“even better than before”).
Plain 比 works fine: 比以前好(多了).

Why use 身体 instead of just saying “she”? Does it mean “body” or “health” here?

身体 here means “health,” not the physical body. Chinese often talks about health in terms of 身体. Also, the possessive 的 is often dropped with close relationships or body-related nouns, so you’ll hear:

她(的)身体比以前好多了。 Because the subject “她” was just mentioned, it’s natural to omit it in the second clause: 现在身体比以前好多了 = “Now her health is much better than before.”

Where can I put 现在? Is its position fixed?

It’s flexible:

现在她身体比以前好多了。
她现在身体比以前好多了。
她身体现在比以前好多了。 (less common but possible) All mean “now her health is much better than before.”

How do I pronounce 比以前 with tone sandhi?

比 is 3rd tone and 以 is also 3rd tone, so 比 changes to 2nd tone before another 3rd tone. Pronounce it as: bí yǐqián (2–3–2).

Could I use 过 to talk about this? For example, 她生过病两天?

No. 过 marks an experienced event without focusing on its time/extent. It doesn’t pair well with a specific finished duration like “two days” in this way. Don’t say 生过病两天. Use the duration patterns with 了 instead:

Completed: 她生病了两天。
Ongoing up to now: 她生病两天了。

Is it okay to say 她病了两天，现在身体好了很多?

Yes. That’s natural. Alternatives expressing similar meaning:

她病了两天，现在身体比以前好得多。
她病了两天，现在好多了。
她病了两天，现在身体好了很多。

How do I say “not as good as before”?

Use 没(有)…那么/像… or just 没(有)…:

现在身体没以前好。
现在身体没有以前那么好。 Avoid 不比以前好 in everyday speech for this meaning; it’s rare and can sound formal or ambiguous.

Does 生病了 mean “got sick” or “is/was sick”?

It can mean either, depending on context:

她生病了两天: the 了 plus duration makes it “was sick for two days.”
她昨天生病了: likely “she got sick yesterday” (onset/change). Context and complements determine whether it’s the onset or the state.

Is the comma necessary? Could it be one sentence?

Chinese often uses a comma to connect two closely related clauses, especially when the second is a result or update:

她生病了两天，现在身体比以前好多了。 You could also split them with a period; both are fine.

Can I say 比起以前 instead of 比以前?

Yes: 现在身体比起以前好多了 is acceptable and a bit more explicit/emphatic. In everyday speech, plain 比以前 is more common and concise.

Are there register or nuance differences among 生病了两天, 生了两天病, and 病了两天?

Slightly:

生病了两天: very common, neutral-modern.
生了两天病: shows the separable nature more explicitly; somewhat bookish but fine.
病了两天: very colloquial and succinct. All are fine in conversation.

Can I strengthen or soften the “much better” part?

Yes:

Stronger: 好太多了 / 好多很多了 (very emphatic, the first is most idiomatic)
Neutral: 好得多 / 好很多 / 好多了
Softer: 好一些 / 好一点(儿) (a bit better)