Kad bi kamp bio bučan, koristili bismo čepiće za uši.

That is a very common question.

In Croatian, kad by itself often means when, but kad bi is a common way to introduce a hypothetical condition, so here it works like if in English.

So:

• Kad pada kiša... = When it rains...
• Kad bi padala kiša... = If it were raining...

In this sentence, Kad bi kamp bio bučan... means If the camp were noisy..., not When the camp is noisy...

You may also see Ako bi in similar sentences, but kad bi is very natural in this kind of unreal or imagined situation.

Because Croatian often uses the conditional in both parts of this kind of sentence:

• the if-clause: Kad bi kamp bio bučan
• the main clause: koristili bismo čepiće za uši

This is normal for a hypothetical situation.

Structure:

• bi + past participle in the first clause
• past participle + bih/bi/bismo/biste/bi in the second clause

So the sentence literally has the pattern:

• If the camp would-be noisy, we would use earplugs

That is not how English says it, but it is how Croatian builds this type of conditional.

Yes, bio is the past participle of biti (to be), but in Croatian that same participle is also used to form the conditional.

So here bio does not mean simple past was by itself. Together with bi, it means would be.

• bio je = he/it was
• bio bi = he/it would be

Since kamp is masculine singular, the participle is bio.

If the subject were feminine, it would change:

• Kad bi soba bila bučna... = If the room were noisy...

Because adjectives in Croatian must agree with the noun they describe.

Here the noun is kamp, which is:

• masculine
• singular
• in the part kamp bio bučan, effectively in the form used with to be

So the adjective must also be masculine singular:

• kamp → bučan

Compare:

• kamp je bučan = the camp is noisy
• soba je bučna = the room is noisy
• mjesto je bučno = the place is noisy

So bučan matches kamp.

This is about clitic placement, which is very important in Croatian.

The word bismo is a clitic, and clitics usually do not stand in the first position of a clause. Because of that, Croatian often puts the participle first:

• koristili bismo

This is the most neutral order here.

You can also say:

• Mi bismo koristili čepiće za uši.

That works because now bismo is not the first element; Mi comes before it.

So:

• Koristili bismo... = very natural
• Mi bismo koristili... = also natural
• Bismo koristili... = generally not acceptable as a neutral standalone clause

Bismo is the 1st person plural conditional auxiliary of biti.

It corresponds to would with we.

The conditional auxiliary forms are:

• bih = I would
• bi = you/he/she/it would
• bismo = we would
• biste = you (plural) would
• bi = they would

So:

• koristio bih = I would use
• koristili bismo = we would use

In your sentence, bismo tells you the subject is we, even though mi is not stated.

Because it agrees with the implied subject we.

In Croatian, the participle in the conditional agrees with the subject in number and, in the singular, also often in gender.

Here the subject is understood as we, so the participle is plural:

• koristili bismo = we would use

If the speaker were a singular masculine speaker:

• koristio bih = I would use

If the speaker were a singular feminine speaker:

• koristila bih = I would use

So the -li in koristili matches a plural subject.

Because čepiće is the accusative plural, which is needed here because it is the direct object of koristili bismo.

Base form:

• čepić = earplug
• čepići = earplugs (nominative plural)

But after a verb like use, Croatian needs the object in the accusative:

• koristiti što? = to use what?
• čepiće

So:

• čepići = earplugs as a subject
• čepiće = earplugs as an object

Example:

• Čepići su skupi. = Earplugs are expensive.
• Kupili smo čepiće. = We bought earplugs.

Because čepić can mean different kinds of small plugs, so za uši specifies that these are earplugs.

Literally:

• čepići = little plugs / plugs
• za uši = for ears

Together:

• čepići za uši = earplugs

In context, Croatians may sometimes shorten it to just čepići, but the full phrase is very common and clear.

After za in this meaning, Croatian uses the accusative.

So:

• za + accusative

The noun uho (ear) has an irregular plural:

• singular: uho
• plural: uši

In this phrase, uši is the accusative plural form.

So:

• za uši = for ears

This is just the normal structure of the expression čepići za uši.

Yes, kamp is a normal and common Croatian word, borrowed long ago and fully natural in modern usage.

Depending on context, it can mean:

• camp
• campground
• campsite
• a camping area in general

So in this sentence, kamp is perfectly natural.

Yes. That is also grammatical and natural.

Both of these can introduce a hypothetical condition:

• Kad bi kamp bio bučan...
• Ako bi kamp bio bučan...

Very roughly:

• ako is the more direct if
• kad bi is also very common in hypothetical statements and can sound slightly more idiomatic in some contexts

For a learner, both are useful and correct here.

It sounds more hypothetical or unreal/imagined than a simple real future possibility.

The conditional forms:

• kad bi ...
• bismo ...

show that the speaker is talking about what would happen under that condition, not necessarily what is expected to happen.

If you wanted a more real, likely condition, Croatian often uses present/future-style forms instead. For example:

• Ako kamp bude bučan, koristit ćemo čepiće za uši. = If the camp is noisy, we’ll use earplugs.

So your original sentence is more like:

• If the camp were noisy, we would use earplugs.

You can add mi, but you usually do not need to.

Croatian often leaves subject pronouns out because the verb form already shows the person and number:

• koristili bismo already means we would use

So the neutral version is:

• Kad bi kamp bio bučan, koristili bismo čepiće za uši.

If you want extra emphasis or contrast, you can say:

• Kad bi kamp bio bučan, mi bismo koristili čepiće za uši.

That would sound more like we would use earplugs as opposed to someone else.

Yes, historically it is.

It comes from čep (plug, stopper) plus the diminutive ending -ić, so literally it is something like small plug.

But in modern usage, čepić is a normal everyday word, and in čepići za uši it simply means earplugs. Learners do not need to feel that it is especially cute or emotional here; it is just the standard word.