Skladištenje u centru za sakupljanje je sigurno.

Skladištenje is a verbal noun: a noun made from a verb. It means storage or storing.

• related verb: skladištiti = to store
• noun: skladištenje = storage / storing

In Serbian, words ending in -nje are very often verbal nouns. They behave like regular nouns, so they can be the subject of a sentence, as they are here.

Because both are verbal nouns.

• skladištenje = storage / storing
• sakupljanje = collecting / collection

This is very common in Serbian. English often uses -ing forms or abstract nouns, while Serbian often uses -nje nouns.

So in this sentence:

• skladištenje names the activity of storing
• sakupljanje names the activity/purpose of collecting

Because sigurno agrees with skladištenje, which is neuter singular.

In Serbian, adjectives must agree with the noun in gender and number.

• masculine: siguran
• feminine: sigurna
• neuter: sigurno

Since skladištenje is neuter, the predicate adjective must also be neuter:

• Skladištenje je sigurno. = Storage is safe.

Because u can take different cases depending on the meaning.

Here it means in / at a location, so it uses the locative case:

• centar = basic form
• u centru = in / at the center

If u expresses movement into somewhere, then Serbian uses the accusative instead:

• Idem u centar. = I’m going to the center.
• Ja sam u centru. = I’m in the center.

So in your sentence, there is no movement; it is just stating location.

Za sakupljanje describes what kind of center it is: a center for collecting.

So:

• centar = center
• centar za sakupljanje = center for collection / collection center

The preposition za often means for and can express purpose. That is exactly what it does here.

It is in the accusative singular, because za normally takes the accusative when it means for.

Here is the useful detail: for many neuter singular nouns, especially ones in -nje, the nominative and accusative look the same.

So:

• nominative: sakupljanje
• accusative: sakupljanje

The form does not change, but the case is still accusative because of za.

Because Serbian has no articles. There is no direct equivalent of English a/an or the.

So Serbian simply says:

• u centru literally = in center

But in natural English, you often translate it as:

• in the center
• at the center

You understand whether it is definite or indefinite from the context.

Either can work, depending on the context and the most natural English phrasing.

• u centru literally means in the center
• but in English, with places like facilities or institutions, at the center is often more natural

So both of these can match the Serbian:

• Storage in the collection center is safe.
• Storage at the collection center is safe.

The Serbian itself uses u because that is the normal preposition for being located inside or at such a place.

Yes, in a normal standard sentence, je is needed.

It is the 3rd person singular present of biti (to be):

• je = is

So:

• Skladištenje je sigurno. = Storage is safe.

Unlike Russian, Serbian normally keeps the present-tense copula in this kind of sentence.

Here it is an adjective, not an adverb.

It describes the subject skladištenje and agrees with it in gender and number, so it means safe.

Compare:

• Skladištenje je sigurno. = Storage is safe.
  → sigurno = adjective

But Serbian sigurno can also be an adverb meaning certainly / surely in other sentences, for example:

• On će sigurno doći. = He will certainly come.

So the form is the same, but the function is different.

Yes, Serbian word order is fairly flexible, but changing it changes the focus or emphasis.

The original sentence:

• Skladištenje u centru za sakupljanje je sigurno.

This sounds natural and presents storage at the collection center as the topic.

A different order is possible, for example:

• Sigurno je skladištenje u centru za sakupljanje.

But that sounds more marked and less neutral.

So the original order is a good standard way to say it.