Kad uđeš u ured, otkopčaj kaput.

Kad and kada both mean when.

In this sentence, kad introduces a time clause: Kad uđeš u ured... = When you enter the office...

The difference is mostly stylistic:

• kad = shorter, very common in everyday speech
• kada = a bit more formal or emphatic

So:

• Kad uđeš u ured, otkopčaj kaput.
• Kada uđeš u ured, otkopčaj kaput.

Both are correct and mean the same thing here.

This is mainly about aspect.

Croatian verbs often come in pairs:

• ulaziti = to be entering / to enter habitually (imperfective)
• ući = to enter as a completed action (perfective)

Here, uđeš is from ući and is perfective. It fits well because the sentence means:

• first, you enter
• then, you unbutton your coat

So the structure is about a completed event before another action:

• Kad uđeš u ured, otkopčaj kaput. = When you’ve entered / once you enter the office, unbutton your coat.

If you said kad ulaziš, it would sound more like when you are entering or whenever you enter, which is a bit different.

Uđeš is the 2nd person singular present tense form of the perfective verb ući.

So:

• ja uđem = I enter
• ti uđeš = you enter
• on/ona/ono uđe = he/she/it enters

Even though it is a present tense form, after kad it often refers to a future situation in English:

• Kad uđeš... = When you enter...

This is normal in Croatian.

Because u can take different cases depending on meaning.

With u:

• u + accusative = movement into something
• u + locative = location in something

Here, entering involves movement toward the inside of the office, so Croatian uses accusative:

• ured → accusative singular: ured
• u ured = into the office

Compare:

• Uđeš u ured. = You enter the office.
• Sjediš u uredu. = You are sitting in the office.

So:

• u ured = direction/movement
• u uredu = location

Because ured is a masculine inanimate noun, and many masculine inanimate nouns have:

• nominative singular = same as
• accusative singular

So:

• ured = office
• u ured = into the office

The form does not change, even though the case changes.

This is very common with masculine inanimate nouns.

Otkopčaj means unbutton! or more literally undo/unfasten!

It is the imperative, 2nd person singular, of otkopčati.

So the sentence is giving a direct instruction to one person:

• otkopčaj kaput = unbutton your coat

Croatian often uses the imperative very naturally in instructions and everyday commands.

Croatian often leaves out possessives like my, your, his, etc. when the meaning is obvious from context.

Here, if someone is being told:

• Kad uđeš u ured, otkopčaj kaput.

it is naturally understood as your coat.

Adding svoj is possible, but it would usually sound more explicit than necessary:

• otkopčaj svoj kaput = unbutton your own coat

In normal speech, otkopčaj kaput is the more natural version.

Because Kad uđeš u ured is a dependent clause that comes before the main clause.

Structure:

• Kad uđeš u ured = subordinate time clause
• otkopčaj kaput = main clause

Croatian normally separates that kind of clause with a comma:

• Kad uđeš u ured, otkopčaj kaput.

This is similar to English:

• When you enter the office, unbutton your coat.

Yes. Croatian word order is more flexible than English, though some versions sound more natural than others.

The original sentence:

• Kad uđeš u ured, otkopčaj kaput.

is very natural.

You could also say:

• Otkopčaj kaput kad uđeš u ured.

This still means the same thing: Unbutton your coat when you enter the office.

The difference is mostly about emphasis:

• clause first = sets the time/background first
• main command first = emphasizes the action a bit more

It can work as either, depending on context.

It might mean:

1. a specific future instruction
   • When you go into the office, unbutton your coat.

or

2. a general instruction/habit
   • Whenever you enter the office, unbutton your coat.

Croatian often uses the same structure for both, and context tells you which is meant.

A few sounds here are especially important:

• đ in uđeš sounds roughly like the j in jam, but softer
• č in otkopčaj sounds like ch in church
• j in otkopčaj sounds like English y in yes

Very approximate pronunciation:

• uđeš ≈ OO-jesh (with a softer j sound than English)
• otkopčaj ≈ ot-KOP-chai

Also remember that Croatian spelling is quite phonetic, so words are usually pronounced very close to how they are written.