Djeca su selotejpom lijepila fotografije na zid, a učiteljica je uzela klamericu za dokumente.

Djeca means children and is treated as a plural subject.

In the past tense, Croatian uses:

• a form of biti (sam, si, je, smo, ste, su)
• plus the past participle

So:

• Djeca su lijepila = The children were sticking / stuck

The form lijepila may look confusing, because it can resemble other forms, but here the auxiliary su clearly shows that the subject is plural.

Because selotejpom is in the instrumental case.

The instrumental is often used to show the tool, means, or instrument used to do something.

So:

• lijepiti selotejpom = to stick something using tape
• literally, something like with tape

This is very common in Croatian:

• pisati olovkom = to write with a pencil
• rezati nožem = to cut with a knife
• selotejpom = with tape

Usually, for a tool or means, Croatian prefers the bare instrumental without a preposition:

• selotejpom
• olovkom
• rukom

Using sa here would sound less natural if you mean using tape as a tool.

Compare:

• Došao je sa sestrom. = He came with his sister.
  Here sa means together with.
• Lijepili su selotejpom. = They were sticking it with tape.
  Here the instrumental shows the means.

Because fotografije is the direct object of lijepila.

They were sticking what?

• fotografije = photographs

That means it must be in the accusative plural.

For feminine inanimate nouns like fotografija, the nominative plural and accusative plural are the same:

• nominative plural: fotografije
• accusative plural: fotografije

So the form does not change here, even though the case is accusative.

This is a very important Croatian pattern:

• na + accusative = movement onto something
• na + locative = location on something

Here the children are putting the photos onto the wall, so Croatian uses:

• na zid

Compare:

• Stavili su fotografije na zid. = They put the photos onto the wall.
• Fotografije su na zidu. = The photos are on the wall.

So:

• na zid = direction / destination
• na zidu = location

This is about aspect, which is very important in Croatian.

• lijepiti is imperfective: it focuses on the process, repeated action, or ongoing activity
• uzeti is perfective: it focuses on a completed action

In this sentence:

• Djeca su ... lijepila fotografije suggests an ongoing or repeated activity: the children were busy sticking photos up
• učiteljica je uzela klamericu presents one completed action: the teacher took the stapler

So the aspect choice matches the situation very naturally.

It could, but it would change the nuance.

• lijepila = they were sticking / they stuck, with focus on the activity
• zalijepila = they stuck them on successfully / completed sticking them on

So:

• Djeca su lijepila fotografije na zid sounds like an activity in progress
• Djeca su zalijepila fotografije na zid sounds more like the task was completed

Both are possible in different contexts, but the original sentence emphasizes the action as an ongoing classroom activity.

Because uzela is perfective and means a single completed action: she took.

• uzela = she took
• uzimala = she was taking / used to take / kept taking

In the sentence, the teacher performs one clear action at that moment, so uzela is the natural choice.

Because klamericu is the accusative singular form.

The noun is:

• nominative: klamerica = stapler

But as the direct object after uzela (took), it changes to accusative:

• uzela je klamericu

This is the regular pattern for many feminine nouns ending in -a:

• knjiga → knjigu
• olovka → olovku
• klamerica → klamericu

Za dokumente means for documents.

The preposition za often takes the accusative and can express:

• purpose
• intended use
• recipient
• goal

Here it tells us what kind of stapler it is:

• klamerica za dokumente = a stapler for documents

The noun dokumenti becomes accusative plural:

• nominative plural: dokumenti
• accusative plural: dokumente
  because it refers to animate/inanimate? Actually for masculine inanimate, accusative plural often matches nominative in some patterns, but dokumenti is not staying the same here because this noun follows the common -i → -e accusative plural pattern in standard Croatian usage after za in this kind of phrase.

For the learner, the key point is:

• za normally requires the accusative
• therefore dokumente, not dokumenti

A can often be translated as and, but it is not exactly the same as English and in every situation.

It often links two clauses while also showing:

• a contrast
• a shift to a different subject
• a parallel action

Here:

• Djeca su ..., a učiteljica je ...

So a works well because the sentence shifts from the children to the teacher.

It can feel like:

• and
• while
• whereas
• sometimes even a mild but

depending on context.

Because they are clitics, and Croatian clitics tend to go near the beginning of the clause, usually in second position.

So:

• Djeca su selotejpom lijepila...
• učiteljica je uzela...

This is a very normal Croatian pattern.

The clitics here are:

• su
• je

They are unstressed helper words, so they usually cannot stand just anywhere in the sentence.

Yes, it is very common in everyday speech.

Selotejp is a widely used word for sticky tape / clear adhesive tape. It comes from a brand-name-type source, much like how some brand names become everyday words in other languages.

Depending on context, you may also see more descriptive expressions such as:

• ljepljiva traka

But selotejp is very common and natural in ordinary conversation.

Croatian past tense here is formed with:

• the present tense of biti (su, je)
• plus the l-participle of the main verb

So:

• su lijepila
• je uzela

This is the standard way to form the past tense in Croatian.

Examples:

• Djeca su lijepila.
• Učiteljica je uzela.
• Mi smo došli.
• Ona je napisala.

So if you see je, su, sam, si, smo, ste plus another verb form, you are very often looking at the past tense.