Learn Hardcore Czech: V pondělí jsem celý den v práci.

Jsem is the 1st person singular present form of být (to be). It already includes the idea of “I”, so Czech usually drops the subject pronoun:

(Já) jsem v práci. = I am at work.

Czech is a “pro‑drop” language: the personal pronoun (já, ty, etc.) is only used when you want to emphasize it (e.g. Já jsem v práci, ne on. – I am at work, not him).

Is this sentence present tense or past tense? How do I translate V pondělí jsem celý den v práci?

Grammatically it’s present tense:

Literal meaning: On Monday I am at work all day.

In natural English, depending on context, you might say:

On Monday I’m at work all day. (talking about a timetable / this coming Monday)
On Mondays I’m at work all day. (habitual schedule – see more below)

It is not past tense; the past would need byl(a):

V pondělí jsem byl celý den v práci. = I was at work all day on Monday.

Can V pondělí jsem celý den v práci also mean “On Mondays I’m at work all day” (regularly)?

Yes, it often can. Czech uses the same form v pondělí both for:

a specific Monday (understood from context)
a repeated situation (regular schedule)

Context usually tells you which is meant.
If you want to be very clear about a repeated habit, you can say:

Každé pondělí jsem celý den v práci. = Every Monday I’m at work all day.

Why is it v pondělí and not na pondělí or something else?

With days of the week, Czech normally uses v:

v pondělí – on Monday
v úterý – on Tuesday
v pátek – on Friday

The preposition v + locative is the standard way to say “on [day]” in Czech.

Na pondělí exists, but it means “for Monday / scheduled for Monday”, e.g.:

Mám to naplánované na pondělí. – I have it planned for Monday.

What case is pondělí in, and why doesn’t it change its form?

In v pondělí, pondělí is in the locative case, because v (in/at) for time expressions takes the locative.

However, pondělí is a type of noun that is almost indeclinable in the singular: most of its singular case forms look the same (pondělí). So nominative, accusative, genitive, dative, locative all appear as pondělí; only the instrumental is pondělím.

So:

pondělí (N) – Monday
v pondělí (L) – on Monday

Look identical, but the function is different.

What exactly does celý den mean? Is it different from just den?

den = (a) day
celý den = the whole day / all day

So:

jsem v práci den – sounds odd, more like “I’m at work a day” (not idiomatic)
jsem v práci celý den – “I’m at work all day”

Celý is an adjective meaning whole / entire and here it agrees with den (masculine animate/inanimate, accusative singular), so the form is celý.

Why is it celý den, not celý denem or something with an ending that looks more like a case?

Den is a masculine noun. In this sentence:

It is the object of the time expression “for how long?” → accusative case.
Accusative singular of den is also den (same form as nominative).

Celý must agree with den in case, gender, and number:

masculine, singular, accusative → celý

So celý den is the correct accusative phrase: the whole day / all day.

Why do we say v práci and not just práci? Can we omit the v?

You cannot omit v here.

v práci = at work (literally: in work / in the workplace) – locative with a preposition
práci alone is just the noun in some case (e.g. accusative, dative), not “at work”.

Saying:

jsem celý den práci

would be wrong; it sounds like “I am the whole day (to) work” with no preposition holding it together.

To express location (at work), you need v + locative: v práci.

What case is práci, and why does práce change to práci?

The base form is práce (work; also “job” in some contexts), a feminine noun.

With the preposition v in a locative phrase (“where?”):

v + locative singular of práce → v práci

The pattern is like many feminine nouns ending in -e/-ě:

kavárna → v kavárně (in the café)
škola → ve škole (at school)
práce → v práci (at work)

So práci here is locative singular.

Can I change the word order, e.g. say Jsem v práci celý den v pondělí? Does it sound natural?

Czech word order is quite flexible, but some choices are more natural.

V pondělí jsem celý den v práci. is very neutral and common.

Other possible orders:

V pondělí jsem v práci celý den. – also fine, slightly different rhythm.
Celý den jsem v pondělí v práci. – emphasizes “the whole day”.

Jsem v práci celý den v pondělí is understandable, but sounds less natural; the clitic verb jsem usually likes to be in second position (after the first stressed phrase), so native speakers prefer it earlier in the sentence, like:

V pondělí jsem celý den v práci.

Why is there no article like “the” or “a” before den or práci?

Czech has no articles (no direct equivalents of a/an/the).

So:

den = a day / the day
práce = work / the work / a job (depending on context)
v práci = at work

The definiteness (a vs the) is understood from context, not marked by a separate word.

Is there a verb meaning “to work” in this sentence? It just looks like “I am at work”.

There is no verb “to work” here; the structure is “to be” + location:

jsem v práci = I am at work (literally: I am in work)

If you want to say “I work all day on Monday” with an explicit verb, you can say:

V pondělí pracuju celý den.
(pracovat = to work)

The meaning is similar, but the focus is slightly different:

jsem celý den v práci – emphasizes being at the workplace all day.
pracuju celý den – emphasizes the activity of working all day.

How do I pronounce V pondělí jsem celý den v práci and where is the stress?

In Czech, stress is always on the first syllable of a word (content words). Short prepositions like v are usually unstressed and cliticized to the next word.

Approximate breakdown (stressed syllables in caps):

(v) PON‑dě‑lí
JSEM
CE‑lý
DEN
(v) PRÁ‑ci

So you’d say something like:
v PON-dye-lee jsem CE-lee den v PRAH-tsi

All vowels are pronounced clearly; é / í / á are long vowels and should be held a bit longer.

How would I say “On Monday I’m not at work all day” or “only half the day”?

You can negate or modify celý den:

V pondělí nejsem celý den v práci.
= On Monday I’m not at work all day.
V pondělí jsem v práci jen půl dne.
= On Monday I’m at work only half the day.
V pondělí jsem v práci jenom odpoledne.
= On Monday I’m at work only in the afternoon.