U genitivu kažemo "bez vremena", a u lokativu "o vremenu".

It is explaining which case form of vrijeme is used after two different prepositions:

• bez vremena = bez
  • genitive
• o vremenu = o
  • locative

So the sentence is about grammar, not about the meaning of time/weather itself.

Because bez always takes the genitive case in standard Croatian.

The basic form is:

• vrijeme = nominative singular

After bez, the noun must change to genitive singular:

• vrijeme → vremena

So:

• correct: bez vremena
• not correct: bez vrijeme

Because o in the sense of about / concerning normally takes the locative case.

So the noun changes from nominative singular:

• vrijeme

to locative singular:

• vremenu

That is why we say:

• o vremenu

The dictionary form is vrijeme.

That is the nominative singular form, the form you would usually find in a vocabulary list.

From that base form, you get:

• nominative singular: vrijeme
• genitive singular: vremena
• locative singular: vremenu

Because vrijeme belongs to a special group of neuter nouns that have an expanded stem in oblique cases.

So instead of simply adding an ending to vrijeme, Croatian uses the stem vremen-:

• vrijeme
• vremena
• vremenu
• vremenom

This pattern is common with nouns like:

• ime → imena
• rame → ramena

So vrijeme is not fully regular in the way a noun like selo is.

Not exactly. Those endings are very common, but the full pattern depends on the noun type.

For vrijeme, the forms are:

• genitive singular: vremena
• locative singular: vremenu

Many neuter nouns also have:

• genitive singular in -a
• locative singular in -u

For example:

• selo → sela → selu

So the case endings are familiar, but vrijeme is still special because of the stem change vrijeme → vremen-.

They literally mean:

• in the genitive
• in the locative

Notice that genitiv and lokativ themselves also change form here:

• genitiv → genitivu
• lokativ → lokativu

That is because they come after u, meaning in, and here u is used with the locative case.

So the grammar terms are being declined too.

No. U can take either the locative or the accusative, depending on meaning.

A simple rule is:

• u + locative = location, being in something
• u + accusative = motion into something

Examples:

• u kući = in the house
• u kuću = into the house

In u genitivu and u lokativu, the idea is in the genitive / in the locative, so Croatian uses the locative.

In Croatian, kažemo is often used in explanations to mean something like:

• we say
• the correct form is
• this is how one says it

It does not have to mean a literal group of people speaking together. It is a very natural way to present grammar or usage.

So:

• U genitivu kažemo bez vremena
  means roughly
• In the genitive, we say bez vremena

Yes, exactly.

One of the most important things in Croatian is that prepositions usually require a specific case. In this sentence:

• bez requires the genitive
• o requires the locative

So when you learn a preposition, it is best to learn it together with the case it governs.

For example:

• bez + genitive
• o + locative
• s(a) + instrumental
• do + genitive

That way, forms like bez vremena and o vremenu become much easier to remember.