Kad se pojavi roj, djeca se malo uplaše, ali im ja kažem da samo mirno stoje.

Kad means when. It is the shorter, very common everyday form of kada. In this sentence, kad and kada would both work:

• Kad se pojavi roj...
• Kada se pojavi roj...

The shorter kad sounds a bit more conversational.

Because the verb is pojaviti se, which means to appear / to show up.

Here, se is part of the verb, not a separate word meaning himself/herself/themselves. Croatian has many verbs that are used with se as part of their normal dictionary form.

So:

• pojaviti se = to appear
• roj se pojavi = a swarm appears

They are present-tense forms, but of perfective verbs:

• pojaviti se = to appear
• uplašiti se = to get scared

In Croatian, perfective present is often used in sentences like this to describe what happens when/whenever one complete event triggers another:

• Kad se pojavi roj, djeca se malo uplaše...

This means something like:

• When/whenever a swarm appears, the children get a little scared...

So although the forms are present tense, the meaning is a repeated sequence of complete events.

Roj means swarm, usually of bees or other insects.

It is in the nominative singular here because it is the subject of se pojavi:

• roj se pojavi = a swarm appears

Croatian has no articles, so roj can mean a swarm or the swarm, depending on context.

Because djeca means children and is grammatically plural, even though its form may look singular to an English speaker.

So you get:

• djeca se uplaše = the children get scared

not a singular verb.

This is a special noun. Its singular is:

• dijete = child

and its plural is:

• djeca = children

The verb uplašiti se means to get scared or to become frightened.

So:

• djeca se uplaše = the children get scared
• djeca se malo uplaše = the children get a little scared

The word malo means a little or somewhat and softens the statement.

This is slightly different from plaše se, which usually describes a state:

• plaše se = they are afraid / they feel scared
• uplaše se = they get scared

Because they belong to two different verbs:

• pojaviti se = to appear
• uplašiti se = to get scared

So the two se words are not repeating the same idea; each one is part of its own verb. This is completely normal in Croatian.

Im means to them. It is the unstressed dative plural pronoun.

So:

• kažem im = I tell them

Croatian uses the dative for the person you tell something to.

By contrast, njih is a form meaning them in other functions, not the normal unstressed dative here. The stressed/full dative form would be njima, but im is the usual clitic form in a sentence like this.

Yes, it could.

Croatian often leaves out subject pronouns because the verb ending already tells you the person:

• kažem = I say / I tell

So ali im kažem is perfectly possible.

When ja is included, it adds emphasis or contrast:

• ali im ja kažem... = but I tell them...

It gives a slight sense of I’m the one who tells them.

Because im is a clitic: a short unstressed word that usually goes very early in the clause, typically in the second position.

So after ali, it is natural to place im early:

• ali im ja kažem

This is very typical Croatian clitic placement.

English speakers often want to say something more like ali ja im kažem, and that can exist depending on rhythm and emphasis, but ali im ja kažem is very natural.

After verbs like reći / kazati / kažem, Croatian very often uses da + present tense to express what someone is told to do.

So:

• kažem da stoje literally looks like I say that they stand
• but in context it means I tell them to stand

This is a normal Croatian structure for an indirect command or instruction.

English prefers:

• I tell them to stand still

Croatian often prefers:

• kažem im da mirno stoje

Word by word:

• samo = just / simply
• mirno = calmly / quietly / still
• stoje = they stand

In this sentence, mirno stoje is best understood as stand still or stay still.

So:

• da samo mirno stoje = to just stand still

The idea is: don’t panic, don’t run around, just stay calm and still.

Because it refers to djeca = the children.

The verb stajati means to stand, and stoje is the 3rd person plural present form:

• oni stoje = they stand

So here:

• da samo mirno stoje = that they should just stand still