V sedm hodin už sedím v kanceláři.

Yes. V sedm hodin means at seven o’clock.

In everyday Czech, v sedm is very common and often sounds more natural in speech.
So these are both fine:

• V sedm hodin už sedím v kanceláři.
• V sedm už sedím v kanceláři.

The longer version with hodin is just more explicit.

Because Czech numerals affect the noun after them.

With clock time, the pattern is:

• jedna hodina
• dvě hodiny
• tři hodiny
• čtyři hodiny
• pět hodin
• šest hodin
• sedm hodin

After 5 and higher, Czech uses the genitive plural of the noun, so hodina becomes hodin.

That is why you get v sedm hodin.

Už means already.

In this sentence it gives the idea that by seven o’clock, I’m already sitting in the office. It often suggests that this is early, or earlier than someone might expect.

So without už:

• V sedm hodin sedím v kanceláři. — At seven I am sitting in the office.

With už:

• V sedm hodin už sedím v kanceláři. — By seven I’m already sitting in the office.

Because sedím specifically means I am sitting or I sit.

Czech often uses a more concrete verb where English might prefer a more general one. So this sentence paints a more vivid picture: not just I am in the office, but I am already sitting there, probably at my desk, working.

Compare:

• Jsem v kanceláři. — I am in the office.
• Sedím v kanceláři. — I am sitting in the office.

The second one is more specific.

Sedím is the 1st person singular present tense of sedět — to sit / to be sitting.

So:

• já sedím — I am sitting
• ty sedíš — you are sitting
• on/ona sedí — he/she is sitting

The ending -ím tells you it is I.

Because sedím and sednu si mean different things.

• sedím = I am sitting / I sit — a state
• sednu si = I will sit down / I’ll take a seat — an action of changing position

In your sentence, the speaker is already in that state at seven, so sedím is correct.

Compare:

• V sedm hodin už sedím v kanceláři. — At seven I’m already sitting in the office.
• V sedm hodin si sednu v kanceláři. — At seven I’ll sit down in the office.

Because after v meaning in, Czech uses the locative case for location.

The base noun is:

• kancelář — office

Its locative singular form is:

• v kanceláři — in the office

So this is a standard location pattern:

• v domě — in the house
• v autě — in the car
• v kanceláři — in the office

Because Czech usually leaves out subject pronouns when they are obvious from the verb form.

Sedím already tells you the subject is I, so já is not necessary.

• Sedím v kanceláři. — I am sitting in the office.

You can add já for emphasis, contrast, or emotion:

• Já už v sedm hodin sedím v kanceláři. — I’m already sitting in the office at seven.

That sounds more emphatic, like contrasting yourself with someone else.

No, Czech word order is flexible, but different orders change the emphasis.

The original sentence is natural and neutral:

• V sedm hodin už sedím v kanceláři.

Other possible orders:

• Už v sedm hodin sedím v kanceláři. — emphasizes how early it is
• V kanceláři už sedím v sedm hodin. — possible, but less neutral and more context-dependent
• Sedím už v sedm hodin v kanceláři. — understandable, but not the most natural neutral order

A good rule: the original version is a very safe choice.

It can do either, depending on context.

Most often, a sentence like this sounds habitual:

• Every day, by seven, I’m already sitting in the office.

But it can also describe a specific situation if the context makes that clear:

• Tomorrow I have a crazy schedule — v sedm hodin už sedím v kanceláři.

So the Czech present tense can work for both general routine and a planned/expected situation.

Because the two v phrases do different jobs:

• v sedm hodin = at seven o’clock — time
• v kanceláři = in the office — place

Even though both use v, they are different constructions.

• In v kanceláři, v means in, and kanceláři is in the locative.
• In v sedm hodin, the phrase is a fixed time expression meaning at seven o’clock.

So you should not expect the same form after every v. The meaning of the phrase matters.

Yes — that is often a very natural English translation.

Literally, sedím is I am sitting, but in real usage the whole sentence often conveys the idea that by seven the speaker is already there, already at work, already seated.

So depending on context, natural English translations could include:

• At seven o’clock I’m already sitting in the office.
• By seven o’clock I’m already in the office.
• By seven I’m already at my desk.

The exact English wording depends on how literal or natural you want to be.