Cjepivo i sterilizacija su potrebni.

Su is the 3rd person plural form of the verb biti (to be) in the present tense.

• je = is
• su = are

Because the subject is two things together — cjepivo i sterilizacija — Croatian treats it as plural, so you need su, not je.

So the structure is:

• cjepivo i sterilizacija = the subject
• su = are
• potrebni = necessary / needed

Because it agrees with the whole subject, and the subject is compound:

• cjepivo = vaccine / vaccination-related noun
• i = and
• sterilizacija = sterilization

When two nouns are joined by i (and), they usually make a plural subject, so the predicate adjective also becomes plural.

That is why you get:

• su = are
• potrebni = necessary/needed, plural

If there were only one noun, the adjective would be singular:

• Cjepivo je potrebno.
• Sterilizacija je potrebna.

This is a very common question, because the two nouns have different genders:

• cjepivo is neuter singular
• sterilizacija is feminine singular

Together, they form a plural subject. In Croatian, when a plural group contains nouns of different genders, the adjective normally goes in the masculine plural form as the default.

So:

• masculine plural = potrebni
• feminine plural = potrebne
• neuter plural = potrebna

Because the subject is a mixed-gender pair, potrebni is the normal form.

They are in the nominative case, because they are the subject of the sentence.

You can tell from the structure:

• Cjepivo i sterilizacija = what the sentence is about
• su potrebni = are necessary

So both nouns are in their dictionary/basic form here.

Croatian does not have articles like English a/an and the.

So:

• cjepivo can mean a vaccine, the vaccine, or simply vaccine/vaccination, depending on context
• sterilizacija can mean a sterilization, the sterilization, or sterilization in general

You understand whether it is definite or indefinite from the situation, not from an article.

Not completely. Croatian word order is more flexible than English word order, but some positions are more natural than others.

The given sentence:

• Cjepivo i sterilizacija su potrebni.

is a normal, neutral way to say it.

You may also see other orders for emphasis or style, for example:

• Potrebni su cjepivo i sterilizacija.

This can sound a bit more like What is needed is vaccination and sterilization.

One important detail: su is a clitic, so it tends to appear in second position in the sentence or clause. That is why it often comes early.

Because su is a clitic, and Croatian clitics usually go in the second position of the clause, but second position does not always mean second word. It often means after the first whole unit.

Here, Cjepivo i sterilizacija acts as one coordinated subject phrase, so su can naturally come after that whole phrase:

• Cjepivo i sterilizacija su potrebni.

That is normal Croatian word order.

Yes. Cjepivo is grammatically singular in this sentence.

Even though the whole subject is plural because of and, each noun itself stays singular:

• cjepivo = singular
• sterilizacija = singular

English does the same thing:

• The vaccine and sterilization are necessary

You do not need to make both nouns plural just because the verb is plural.

Potrebni is an adjective meaning necessary, needed, or required.

Here it is used as a predicate adjective, meaning it comes after the verb to be:

• su potrebni = are necessary

It agrees with the subject in number and, when relevant, gender.

Compare:

• Cjepivo je potrebno. = The vaccine is necessary.
• Sterilizacija je potrebna. = Sterilization is necessary.
• Cjepivo i sterilizacija su potrebni. = Vaccination and sterilization are necessary.

In a normal present-tense statement like this, no — you normally keep su.

Croatian sometimes omits forms of to be in certain styles, headlines, or very informal notes, but in an ordinary full sentence you would say:

• Cjepivo i sterilizacija su potrebni.

not simply

• Cjepivo i sterilizacija potrebni.

The version without su would sound incomplete in standard everyday prose.

This word can look difficult to English speakers because of the cluster cj.

A helpful rough guide is:

• cjepivo ≈ tsye-pee-vo

More specifically:

• c in Croatian is like ts
• j is like English y
• so cj sounds roughly like tsy

The stress is not marked in normal spelling, and learners usually focus first on getting the consonants right.

Yes, depending on the exact meaning.

• cjepivo usually means vaccine
• cijepljenje means vaccination or the act of vaccinating

So if you specifically mean vaccination as a process or procedure, cijepljenje may be a better choice.

Then the sentence would be:

• Cijepljenje i sterilizacija su potrebni.

However, if the original sentence intentionally uses cjepivo, then that noun is still grammatically correct in the structure shown. The main grammar point stays the same: two coordinated nouns give you su + plural adjective.

Yes. Potreban / potrebna / potrebno / potrebni can often be translated in several ways depending on context:

• necessary
• needed
• required

So this sentence could be understood in slightly different English ways depending on the situation. The Croatian grammar remains the same.