U posudu za cvijeće stavite zemlju za cvijeće i humus.

Because u can take different cases depending on meaning:

• u + accusative = movement into something
• u + locative = being in something

Here the action is putting soil into the container, so Croatian uses u posudu.

Compare:

• Stavite zemlju u posudu. = Put the soil into the container.
• Zemlja je u posudi. = The soil is in the container.

Posudu is accusative singular of the noun posuda.

• nominative: posuda
• accusative: posudu

It changes because it is the destination after u with movement.

Stavite is the imperative form of staviti.

It can mean:

• you all put
• put when speaking politely to one person
• the usual instruction form in recipes, manuals, and gardening directions

So this is the kind of form Croatian often uses for instructions.

Related forms:

• stavi = singular informal, to one person
• stavite = plural or polite singular

Staviti is perfective, while stavljati is imperfective.

In instructions, Croatian often prefers the perfective imperative when the action is seen as one complete step:

• stavite = put/add it in
• stavljajte = keep putting / put repeatedly / be in the process of putting

Since this is a single instruction step, stavite is the natural choice.

Za cvijeće literally means for flowers.

The preposition za here expresses purpose or intended use. So:

• posuda za cvijeće = a container/pot for flowers
• zemlja za cvijeće = soil for flowers, meaning potting soil

This is a very common Croatian pattern:

• čaša za vodu = glass for water
• četka za kosu = brush for hair
• hrana za pse = food for dogs

Because it belongs to two different nouns:

• posuda za cvijeće = flower pot / container for flowers
• zemlja za cvijeće = potting soil

Even though English might avoid repeating for flowers, Croatian often keeps both phrases because each one is a separate noun expression.

Here cvijeće is in the accusative, because za normally takes the accusative.

But cvijeće is a neuter singular noun whose nominative and accusative forms are the same:

• nominative: cvijeće
• accusative: cvijeće

So it looks unchanged, even though grammatically it is accusative.

Grammatically, cvijeće is singular neuter, even though its meaning is collective and refers to flowers in general.

That is why you get singular agreement with it.

This is similar to some collective nouns in other languages: one word can refer to many items.

Not usually. Zemlja za cvijeće means potting soil or flower soil, the kind of soil used for potted plants.

So in context it sounds more natural than plain zemlja, which can also mean:

• earth
• soil
• ground
• dirt
• land
• country, in some contexts

The phrase zemlja za cvijeće makes the gardening meaning clear.

Humus in Croatian usually means humus in the gardening/soil sense: organic, nutrient-rich material mixed into soil.

In this sentence, it is something added together with zemlja za cvijeće.

So the phrase means you should put in:

• potting soil
• and humus

Because Croatian has no articles.

So a noun like posuda can mean:

• a container
• the container

The exact meaning depends on context.

That is normal in Croatian, and learners from English often notice it right away.

No, Croatian word order is fairly flexible.

The given sentence is natural and neutral:

• U posudu za cvijeće stavite zemlju za cvijeće i humus.

But other orders are also possible, for example:

• Stavite zemlju za cvijeće i humus u posudu za cvijeće.

Both are correct. The original version sounds like a typical instruction, with the destination u posudu za cvijeće placed first for emphasis or clarity.

Yes. Posuda za cvijeće is understandable and fairly general: a container/pot for flowers.

Depending on region and style, you may also hear more specific everyday words such as:

• tegla
• saksija in some varieties

But posuda za cvijeće is clear, neutral, and works well in instructional language.

Then you would use the singular informal imperative:

• U posudu za cvijeće stavi zemlju za cvijeće i humus.

So:

• stavi = to one person, informal
• stavite = to several people, or to one person politely

That distinction is very common in Croatian imperatives.