Kad budem našla kemijsku, ispisat ću obrazac i potpisati ga.

Because after kad when talking about a future event, standard Croatian normally uses future II in the subordinate clause.

So:

• kad budem našla kemijsku = when I have found / when I find a pen
• ispisat ću obrazac = I will fill out the form

The idea is that first I find the pen, and after that I fill out and sign the form.

This pattern is very common in Croatian:

• Kad budem imala vremena, nazvat ću te. = When I have time / when I get time, I’ll call you.
• Ako bude padala kiša, ostat ćemo doma. = If it rains, we’ll stay home.

So even though English usually says when I find..., Croatian often uses future II here.

Because the speaker is female.

The form našla is the feminine singular past participle, and it agrees with the subject I if the speaker is a woman.

• female speaker: budem našla
• male speaker: budem našao

So a man would say:

• Kad budem našao kemijsku, ispisat ću obrazac i potpisati ga.

This kind of gender agreement is very normal in Croatian in past-participle forms.

Here kemijsku means a ballpoint pen.

It is short for kemijska olovka. In everyday Croatian, people very often shorten this and just say:

• kemijska = ballpoint pen

In this sentence, kemijsku is the accusative form of kemijska because it is the direct object of našla.

So:

• kemijska = a ballpoint pen
• našla sam kemijsku = I found a ballpoint pen

This can confuse English speakers because kemijska looks like it should mean chemical, but in everyday Croatian it commonly means pen.

Both words are direct objects, but they belong to different noun patterns.

• kemijska is a feminine noun/adjective-based form, so in the accusative singular it becomes kemijsku
• obrazac is a masculine inanimate noun, and masculine inanimate nouns usually have the same form in nominative and accusative singular

So:

• kemijska → kemijsku
• obrazac → obrazac

Compare:

• Imam kemijsku. = I have a pen.
• Imam obrazac. = I have a form.

This is a very common Croatian case pattern.

Both are possible in principle, but ispisat ću is a very normal future form in Croatian.

Croatian future I is made with:

• the verb htjeti in a clitic form: ću, ćeš, će, ćemo, ćete, će
• plus the infinitive

When the infinitive comes before the auxiliary, the final -i is dropped:

• ispisati + ću → ispisat ću
• potpisati + ću → potpisat ću

When the auxiliary comes first, the full infinitive stays:

• ću ispisati
• ću potpisati

So these are both future-type structures:

• Ispisat ću obrazac.
• Ću ispisati obrazac. → this is not correct because clitics cannot stand first
• Ja ću ispisati obrazac. → correct
• Obrazac ću ispisati. → correct

The sentence uses the very common pattern infinitive + ću.

Because Croatian often omits the repeated auxiliary when two infinitives share the same future meaning.

So:

• ispisat ću obrazac i potpisati ga

means the same as:

• ispisat ću obrazac i potpisat ću ga

The first version is smoother and more natural because the future auxiliary ću is understood for both verbs.

This is similar to English:

• I’ll fill out the form and sign it.

You do not need to repeat I’ll before sign.

Ga means it, and it refers to obrazac.

So:

• obrazac = form
• potpisati ga = sign it

Grammatically, ga is:

• accusative singular
• masculine

That matches obrazac, which is a masculine noun.

So the structure is:

• ispisat ću obrazac = I’ll fill out the form
• i potpisati ga = and sign it

Because Croatian, like English, usually avoids repeating the noun when the meaning is already clear.

So instead of saying:

• ispisat ću obrazac i potpisati obrazac

Croatian naturally says:

• ispisat ću obrazac i potpisati ga

This sounds more natural and less repetitive.

Yes, in this sentence kad and kada mean the same thing: when.

• Kad budem našla kemijsku...
• Kada budem našla kemijsku...

Both are correct.

The difference is mostly stylistic:

• kad is shorter and very common in everyday speech
• kada can sound a bit more formal or emphatic, depending on context

So here kad is completely normal.

Here it means to fill out a form by writing in it.

The verb ispisati can mean write out, fill in, or complete in writing, depending on context.

With obrazac, it suggests entering the required information on the form.

A related verb is ispuniti:

• ispuniti obrazac = fill out the form
• ispisati obrazac = fill out / write out the form

In many situations, ispuniti obrazac is the more general and very common expression, while ispisati obrazac emphasizes the writing aspect a bit more.

Yes:

• Kad = when
• budem našla = I find / I have found (future-related subordinate clause, feminine speaker)
• kemijsku = a ballpoint pen
• ispisat ću = I will fill out / write out
• obrazac = form
• i = and
• potpisati = sign
• ga = it

So the structure is:

• When I find a pen, I’ll fill out the form and sign it.

This sentence is a good example of several useful Croatian features at once: future II, gender agreement, case endings, future I, and a clitic pronoun.