Ovo je nova vrećica sjemena.

Because ovo here is a standalone demonstrative pronoun meaning this or this thing. It is not directly modifying vrećica.

In Croatian, when the demonstrative stands by itself, ovo je... is a very common pattern for introducing or identifying something: Ovo je... = This is...

If the demonstrative directly went with vrećica, then it would have to agree with that feminine noun: ova vrećica = this bag/packet

So:

• Ovo je nova vrećica sjemena. = This is a new seed packet.
• Ova vrećica sjemena je nova. = This seed packet is new.

That is an important difference in structure.

Je means is. It is the 3rd person singular present-tense form of biti = to be.

So:

• ovo = this
• je = is
• nova vrećica sjemena = a new bag/packet of seed(s)

Also, je is a clitic, which means it normally appears near the beginning of the sentence, often in the second position. That is why Ovo je... sounds natural.

Because nova has to agree with vrećica.

The noun vrećica is:

• feminine
• singular
• here in the nominative

So the adjective must match it:

• masculine singular: novi
• feminine singular: nova
• neuter singular: novo

Since vrećica is feminine, the correct form is nova.

It is in the nominative singular.

In this sentence, vrećica is the predicate noun after je in a sentence identifying what something is:

Ovo je nova vrećica sjemena.
Literally: This is a new packet of seeds.

So vrećica appears in its basic dictionary form, the nominative singular.

Vrećica means a small bag, little bag, packet, or sachet, depending on context.

It is related to vreća, which means bag or sack. The ending -ica often gives a smaller or more specific sense, so vrećica is like small bag / little packet.

In the context of sjemena, a very natural English translation is often seed packet.

Because after a noun like vrećica, Croatian usually puts the thing contained inside it into the genitive.

So the structure is basically:

• vrećica sjemena = a packet of seed / a packet of seeds

English uses of here, but Croatian often just uses the genitive form with no separate word for of.

The base form is sjeme.
In this sentence, it changes to sjemena because of that genitive relationship.

This is a good question, because sjeme has an irregular declension, and in ordinary spelling sjemena can match more than one grammatical form.

In this sentence, the most useful thing for a learner is:

• vrećica sjemena is the normal way to say packet of seed(s)

In practice, the phrase is understood naturally as talking about the contents of the packet, and English may translate it as either seed or seeds, depending on style and context.

So even if the singular/plural issue looks confusing, the key pattern to learn is:

• sjeme = base form
• sjemena after vrećica

Because Croatian does not have articles like English a and the.

So Croatian simply says:

Ovo je nova vrećica sjemena.

Whether English translates that as:

• This is a new seed packet
• This is the new seed packet

depends on context.

Croatian expresses definiteness in other ways, for example through context, word order, or words like ovaj / ova / ovo (this).

Yes, and that is also correct, but the structure and focus are different.

• Ovo je nova vrećica sjemena.
  This introduces or identifies something: This is a new seed packet.

• Ova vrećica sjemena je nova.
  This starts with the noun phrase itself: This seed packet is new.

So the first sentence is more like pointing something out and naming it.
The second sentence is more like talking about this seed packet and saying something about it.

Croatian word order is more flexible than English word order, but the sentence you have is the most neutral and natural version.

Ovo je nova vrećica sjemena. is the standard order.

You can change word order for emphasis, but then the sentence may sound more marked or stylistic. One important thing is that je usually stays near the beginning because it is a clitic.

So for a learner, the best default is to keep this order and use it as a model.