Learn Hardcore Ukrainian: Якщо ти хворий, тобі потрібно відпочити.

Sometimes they can overlap, but in a sentence about a condition like Якщо ти хворий, тобі потрібно відпочити, коли would sound wrong or at least unnatural, because we are not talking about when you are sick, but about the condition if you are sick.

Why is there a comma after хворий?

In Ukrainian, as in English, you usually put a comma between the if-clause and the main clause.
So Якщо ти хворий is one clause, and тобі потрібно відпочити is another. The comma separates them:

Якщо ти хворий, – if you are sick,
тобі потрібно відпочити. – you need to rest.

Why is it ти and not the formal ви in this sentence?

Ти is the informal you (singular), used with friends, family, children, or people your age in casual speech.
Ви is either plural you or polite/formal singular you, used with strangers, older people, or in formal situations.

You could say the formal version as:

Якщо ви хворі, вам потрібно відпочити.
Here ви / хворі / вам are the formal/plural forms.

Why is it хворий and how would this change if the speaker is talking to a woman?

Хворий is the masculine singular form of the adjective sick / ill. It agrees with ти when the listener is male.
If the listener is female, you say:

Якщо ти хвора, тобі потрібно відпочити.

So:

masculine: ти хворий
feminine: ти хвора
The verb потрібно and відпочити do not change with gender here; only the adjective does.

What case is тобі, and why is it used with потрібно?

Тобі is the dative form of ти (to you / for you).
The construction комусь потрібно щось literally means something is needed to someone.

So:

тобі потрібно відпочити = rest is needed to you → you need to rest.
This is a very common impersonal way to express necessity in Ukrainian.

What does потрібно mean here, exactly? Is it like “must” or “need to”?

Потрібно here means need to / should, expressing necessity in a neutral, impersonal way.
It’s softer than a direct must and less personal than ти повинен (you must / you have to).

Approximate nuances:

тобі потрібно відпочити – you need to rest / it’s necessary for you to rest.
ти повинен відпочити (to a man) / ти повинна відпочити (to a woman) – you must / you have to rest (stronger, more direct).
тобі слід відпочити – you ought to rest (a bit formal/literary).

Why is it відпочити and not відпочивати? What’s the difference?

Відпочити is perfective – it focuses on the result or completion: to have (a proper) rest, to get rested.
Відпочивати is imperfective – it focuses on the process or ongoing action: to be resting, to rest (in general).

In this context, тобі потрібно відпочити means “you need to get some rest (so that you’ll be rested)”.
If you used відпочивати, it would sound more like “you need to be resting” or “you need to rest (as an activity)”, which is less natural here.

What is the infinitive form of відпочити, and how would I say “I am resting”?

Both відпочити and відпочивати are infinitives, but of different aspects:

відпочити – perfective infinitive (to rest, achieving the result)
відпочивати – imperfective infinitive (to be resting / to rest habitually)

To say I am resting, you would normally use the imperfective verb:

Я відпочиваю. – I am resting / I rest.

Can the word order be changed? For example, can I say Тобі потрібно відпочити, якщо ти хворий?

Yes, you can change the word order without changing the meaning significantly.
Both are correct:

Якщо ти хворий, тобі потрібно відпочити.
Тобі потрібно відпочити, якщо ти хворий.

The first puts more emphasis on the condition (if you are sick), the second starts with the advice/necessity (you need to rest) and adds the condition after.
Other rearrangements like Тобі відпочити потрібно are possible but sound more marked or stylistic; the original is the most neutral.

Can I drop the pronoun ти and just say Якщо хворий, тобі потрібно відпочити?

Yes, in spoken language people can say Якщо хворий, тобі потрібно відпочити, and it will be understood, especially from context.
However, it’s more natural and clear to keep ти in a simple sentence like this:

Якщо ти хворий, тобі потрібно відпочити.

Dropping the pronoun is more common when the subject is very obvious or has already been mentioned.

Is there any difference between Якщо ти хворий and Якщо ти захворів?

Yes:

Якщо ти хворий – if you are sick (describes your current state).
Якщо ти захворів – if you (have) fallen ill / if you got sick (focuses on the event of becoming ill).

Both could be followed by тобі потрібно відпочити, but the nuance is slightly different:

Якщо ти захворів, тобі потрібно відпочити. – if you have come down ill / if you’ve caught something, you need to rest.

How do you pronounce each word in this sentence?

Approximate pronunciation with stress marked by ´:

Якщо́ – yak-SHÓ
ти – ty (like English “ti” in “tip”, short)
хво́рий – HVO-ryy (the хв is like hv, and и is a short “ih” sound)
тобі́ – to-BÍ
по́трібно – PÓ-treeb-no (stress on по́, і like “ee”)
відпочи́ти – veed-po-CHÝ-ty (stress on чи́; в is like English “v”).

Is this sentence formal or informal in tone?

The use of ти makes it clearly informal. It sounds like advice to a friend, family member, or child.
A more formal/polite version would use ви and the plural adjective:

Якщо ви хворі, вам потрібно відпочити.

The construction вам потрібно is still neutral and polite, not rude or harsh.