Learn Hardcore Czech: Zítra budu odpočívat doma s rodinou. - Tomorrow I will relax at home with my family.

zítra – tomorrow
budu – I will be (1st person singular future of být = to be)
odpočívat – to rest, to relax (imperfective infinitive)
doma – at home
s – with
rodinou – (the) family, in the instrumental case (literally with family)

So the structure is roughly: Tomorrow I will be rest(ing) at home with (the/my) family.
Czech doesn’t need a separate word for I, because budu already tells you the subject is I.

Why do we say budu odpočívat and not just odpočívám for the future?

Czech usually forms the future of imperfective verbs with future of být (to be) + infinitive.

odpočívat (to rest) is imperfective.
Its common future is budu odpočívat = I will be resting / I will rest.

Using present tense odpočívám with a future time word (zítra) is possible in some verbs (especially motion or scheduled events, e.g. zítra jedu do Prahy – I’m going to Prague tomorrow), but for odpočívat it would sound strange in normal speech.
So:

Zítra budu odpočívat. ✅ natural
Zítra odpočívám. ❌ sounds off / unnatural in standard Czech

How is the future of odpočívat formed for all persons?

You combine the future of být with the infinitive odpočívat:

já budu odpočívat – I will rest
ty budeš odpočívat – you (sg., informal) will rest
on / ona / ono bude odpočívat – he / she / it will rest
my budeme odpočívat – we will rest
vy budete odpočívat – you (pl. or formal) will rest
oni budou odpočívat – they will rest

The infinitive odpočívat never changes; only být (budu, budeš, bude…) changes.

What is the difference between odpočívat and odpočinout si / odpočinu?

This is about aspect:

odpočívat – imperfective: focuses on the process / duration of resting
- Zítra budu odpočívat. – I’ll be resting / I’ll spend time resting.
odpočinout si (perfective infinitive) / odpočinu si (1st person) – focuses on the single completed act of getting rest
- Zítra si odpočinu. – I’ll (manage to) get some rest / I’ll have a rest (once, to completion).

So:

Use odpočívat if you think of relaxing over some time.
Use odpočinout si / odpočinu si if you mean “to take a rest” and be done with it.

Can I say Zítra si odpočinu doma s rodinou instead? How is it different?

Yes, that sentence is correct:

Zítra si odpočinu doma s rodinou.

Differences in nuance:

Zítra budu odpočívat… – emphasizes the ongoing activity or period of relaxing during tomorrow.
Zítra si odpočinu… – emphasizes that you will succeed in getting some rest, a more single, completed rest.

Both are natural. The original feels more like “I’ll be relaxing tomorrow,” the alternative a bit more like “I’ll get a chance to rest tomorrow.”

Why is doma used instead of v domě or v domu?

doma is a special adverb meaning at home.

doma – at home (your home, home in general)
v domě – in the house / in a house (literally “in the building”)
v domu – also “in the house”, but this form is less common and more formal/literary today.

In this context you mean at home, not “inside the house (as a physical building)” in contrast to, say, in the garden or garage.
So doma is the natural, everyday choice:

Zítra budu odpočívat doma. – I’ll rest at home tomorrow.

Why is there no word for my before rodinou?

Czech often omits possessive pronouns (my, your, his, etc.) when the owner is clear from context.

Here, if you say:

Zítra budu odpočívat doma s rodinou.

a normal listener will automatically understand with my family, because:

You are talking about your own plans (budu odpočívat = I will rest).
Spending time “with family” is very typically with your own family.

You can add it:

Zítra budu odpočívat doma s mojí rodinou.

This is correct but usually only used if you want to stress whose family (e.g. to contrast it with someone else’s). In neutral context, s rodinou alone is preferred.

What case is rodinou, and why does it have that ending?

rodinou is the instrumental singular of rodina (family).

Nominative singular (dictionary form):

rodina – a/the family

Instrumental singular (feminine, hard-type):

rodina → rodinou

We use the instrumental case after the preposition s when it means with (someone):

s kým? – with whom?
s rodinou – with (the) family
s bratrem – with (my) brother
s kamarádkou – with (my) (female) friend

So s + rodinou is required by grammar: s + instrumental.

Does s rodinou always mean “with my family”, or can it mean “with a family / with some family”?

The phrase s rodinou by itself can be understood in several ways, depending on context, because Czech has no articles (no “the”, “a”).

Typical readings:

In a sentence about yourself and your free time (budu odpočívat doma s rodinou), it almost always means with my family.
In another context, it could be with the family (some specific family everyone knows about in the conversation).
Or more generally with a family (not specified which), but then context has to make that clear.

If you want to be explicit, you can say:

s mojí rodinou – with my family
s rodinou kamaráda – with my friend’s family
s jednou rodinou – with one family / with a certain family

But in the original sentence the default, natural interpretation is with my family.

Where can zítra go in the sentence? Is the word order fixed?

Czech word order is relatively flexible. All of these are grammatically correct:

Zítra budu odpočívat doma s rodinou. – neutral, very natural.
Budu zítra odpočívat doma s rodinou. – also fine; light emphasis on budu (the fact it will happen tomorrow).
Budu odpočívat zítra doma s rodinou. – emphasizes zítra a bit more.
Doma zítra budu odpočívat s rodinou. – emphasizes doma (at home) as the starting point.

In everyday speech, time expressions like zítra very often come at the beginning:

Zítra budu… / Zítra jedu… / Zítra pracuju…

So the original word order is the most neutral and typical.

Where is the subject “I” in Zítra budu odpočívat doma s rodinou?

The subject I is built into the verb form budu.

budu – I will be
budeš – you (sg.) will be
bude – he / she / it will be
etc.

Czech is a pro-drop language: it usually leaves out the personal pronoun (já, ty, etc.) when the verb ending already makes the subject clear.

You could say:

Já zítra budu odpočívat doma s rodinou.

This is correct, but já adds emphasis, roughly like “I, personally, will rest tomorrow…”. In neutral speech, you simply leave já out.

How do you pronounce Zítra budu odpočívat doma s rodinou?

Approximate pronunciation (IPA-like):

zítra – [ˈziːtra]
budu – [ˈbudu]
odpočívat – [ˈotpot͡ʃiːvat]
doma – [ˈdoma]
s rodinou – [s ˈrodɪnou]

Whole sentence:
[ˈziːtra ˈbudu ˈotpot͡ʃiːvat ˈdoma s ˈrodɪnou]

Notes:

Stress is always on the first syllable of each word: ZÍ‑tra BU‑du OD‑po‑čí‑vat DO‑ma …
č is like ch in “chat”.
í is a long ee sound.
ou in rodinou is like “o” in “go”, but a bit shorter.

Can I use relaxovat instead of odpočívat?

Yes, you can also say:

Zítra budu relaxovat doma s rodinou.

Differences:

odpočívat – native Slavic verb, fully standard, neutral.
relaxovat – a borrowing from English (to relax), common in everyday speech, slightly more colloquial / modern in feel, but still acceptable standard Czech.

Semantically in this sentence, they are almost the same: I will be relaxing at home with (my) family tomorrow.