Semina in terra bona crescunt.

Because semina is plural.

The dictionary form is semen, meaning seed.
Its plural is semina, meaning seeds.

This is a very common pattern for a neuter third-declension noun:

• semen = seed
• semina = seeds

A native English speaker may find this surprising because -a often looks singular at first, but in Latin it is very often a neuter plural ending.

Because semina is a neuter plural noun.

In Latin, many neuter nouns have:

• singular nominative/accusative ending in -um or another neuter form
• plural nominative/accusative ending in -a

For semen, the nominative plural is semina.

So here:

• semina = the seeds / seeds

Even though -a often makes English-speaking learners think of a feminine singular noun like terra, here it is actually a neuter plural form.

Because it agrees with the subject, semina.

Since semina means seeds and is plural, the verb must also be plural:

• crescit = it grows
• crescunt = they grow

So Semina ... crescunt means The seeds grow or Seeds grow.

Crescunt is:

• present tense
• active voice
• indicative mood
• third person plural

It comes from the verb crescere, meaning to grow.

So crescunt can be translated as:

• grow
• are growing
• sometimes do grow, depending on context

Because in can take two different cases, depending on meaning.

• in
  • ablative = in/on something, showing location
• in
  • accusative = into/onto something, showing motion toward

Here the meaning is location: the seeds grow in good soil/earth.
So Latin uses the ablative:

• in terra bona = in good soil / in good earth

If the idea were motion into something, you would expect the accusative instead:

• in terram bonam = into good soil

Because they match in case, number, and gender.

• terra is feminine singular ablative
• bona is also feminine singular ablative

That tells you that bona describes terra, not semina.

So:

• terra bona = good soil / good earth

It cannot be modifying semina, because semina is neuter plural, and bona here is not neuter plural.

No, not in this sentence.

If bona were describing semina, it would have to be neuter plural nominative, and the form would also be bona. So the form alone might look ambiguous at first.

But the phrase in terra bona makes the structure clear:

• in with location takes the ablative
• terra is ablative singular
• bona matches terra

So here bona means good and describes terra, not semina.

Terra can mean several related things, including:

• earth
• land
• ground
• sometimes soil

In this sentence, English often uses soil because that sounds natural with seeds growing. So in terra bona is commonly understood as:

• in good soil

But the Latin word itself is broader than just soil.

Because Latin has no articles.

Latin does not have separate words for:

• the
• a/an

So semina can mean:

• seeds
• the seeds

And terra bona can mean:

• good soil
• the good soil

You decide from context which English translation sounds best.

The basic meaning does not depend mainly on word order, because the endings show the grammar.

So these would all mean roughly the same thing:

• Semina in terra bona crescunt.
• In terra bona semina crescunt.
• Semina crescunt in terra bona.

Latin word order is more flexible than English word order.
Writers often change the order for:

• emphasis
• style
• rhythm

In this sentence, the order is straightforward and natural.

Because Latin adjectives can come before or after the noun.

So both of these are possible:

• terra bona
• bona terra

Both mean good soil/earth.

Latin often places adjectives after nouns, especially in simple textbook sentences, but there is no rule that they must come after. Word order can shift for style or emphasis.

It is nominative plural.

We know this because semina is the subject of crescunt:

• semina = the seeds
• crescunt = grow

For neuter nouns, the nominative plural and accusative plural are often the same form, but here the role in the sentence shows that it is nominative.

It is ablative singular.

More exactly:

• terra = ablative singular
• bona = ablative singular feminine, agreeing with terra

The ablative is used because in here expresses location:

• in terra bona = in good soil

They are:

• semen, seminis = seed
• terra, terrae = earth, land, ground
• bonus, bona, bonum = good
• cresco, crescere, crevi, cretum = grow

A learner should especially notice that semina comes from semen, not from a first-declension noun.

A common classroom pronunciation for Classical Latin would be roughly:

SEH-mi-na in TEHR-ra BOH-na KREHS-koont

A few useful points:

• c in crescunt is always hard, like k
• vowel sounds are usually pure, not gliding as much as in English
• r is trilled or tapped
• ae is not in this sentence, but in Classical Latin it sounds roughly like ai in aisle

So crescunt does not sound like English crescent at the beginning; it begins with a hard kr sound.

Yes.

The Latin present tense often covers both ideas:

• they grow
• they are growing

Which translation is best depends on context.

In a general statement, English usually prefers grow:

• Seeds grow in good soil.

If you were describing something happening right now, are growing could also work:

• The seeds are growing in good soil.

Yes.

It has everything needed for a full sentence:

• subject: Semina
• verb: crescunt
• prepositional phrase: in terra bona

So it is a complete statement: the seeds grow in good soil.