V půl osmé už jsem v kanceláři.

This is one of the most common surprises for English speakers.

In Czech, v půl osmé literally means something like at half of the eighth hour. So it refers to the time halfway to eight, which is 7:30.

A few examples:

• v půl páté = 4:30
• v půl osmé = 7:30
• v půl deváté = 8:30

So Czech counts this expression toward the next hour, not the current one.

Because this expression does not use the cardinal number osm (eight). It uses an ordinal idea: the eighth hour.

So:

• osm = eight
• osmá = eighth

After půl, Czech uses the genitive form, so osmá becomes osmé.

That is why you say:

• v půl osmé
  not
• v půl osm

The structure is:

• v = at
• půl = half
• osmé = of the eighth

More literally, it is shorthand for something like v půl osmé hodiny.

The hidden noun is hodina (hour), which is feminine, and the ordinal agrees with that idea. Because půl requires the genitive, you get osmé.

You do not need to build this from scratch every time, but it helps to know that this is a fixed Czech time pattern.

Because Czech usually drops subject pronouns when they are not needed.

The verb jsem already tells you the subject is I:

• jsem = I am

So V půl osmé už jsem v kanceláři naturally means At 7:30 I’m already in the office without needing já.

You could add já, but then it sounds more emphatic or contrastive:

• V půl osmé už jsem já v kanceláři.

That would suggest something like I’m already in the office at 7:30, unlike someone else.

Here už means already.

It adds the idea that by that time, the speaker is in the office sooner than you might expect, or that this is already true at 7:30.

So the sentence is not just a neutral statement about location and time. It has a nuance like:

• By 7:30, I’m already in the office.
• I’m in the office as early as 7:30.

Without už, the sentence would be more neutral:

• V půl osmé jsem v kanceláři.
  = At 7:30 I’m in the office.

Because after v when you mean location, Czech uses the locative case.

The basic form is:

• kancelář = office

But after v for in/at a place, it becomes:

• v kanceláři = in the office / at the office

So this is a normal case change:

• kancelář → v kanceláři

It can mean either, depending on context.

Czech uses v kanceláři literally as in the office, but in English the most natural translation is often at the office.

So:

• Jsem v kanceláři.
  can mean
  
  • I’m in the office.
  • I’m at the office.

English chooses between in and at more explicitly than Czech does here.

Czech word order is flexible, but this order is very natural.

It does a few things:

• V půl osmé puts the time first
• už highlights already
• jsem v kanceláři gives the main statement

So the sentence flows as:

At 7:30, already, I am in the office.

A different order is also possible, for example:

• V půl osmé jsem už v kanceláři.

That is also natural and very close in meaning. The difference is mainly about emphasis and rhythm, not basic meaning.

Czech has no articles, so there is no separate word for the or a/an.

That means:

• kancelář can mean an office or the office
• v kanceláři can mean in an office or in the office

The context tells you which one is meant. In this sentence, English would usually say the office because that is the natural meaning in context.

Here jsem simply means am.

This sentence is present tense:

• jsem v kanceláři = I am in the office

It is not past tense, because there is no past participle like byl.

Compare:

• jsem v kanceláři = I am in the office
• byl jsem v kanceláři = I was in the office

So in your sentence, jsem is just the present-tense form of to be.