Avia dicit rosas et lilia ex bona terra nasci.

Avia is nominative singular. It is the subject of dicit.

So the basic main clause is:

Avia dicit = Grandmother says

The rest of the sentence tells us what she says.

Because its subject, avia, is singular.

• dicit = she says
• If the subject were plural, you would expect a plural verb, such as dicunt = they say

So avia dicit matches correctly: one grandmother, one singular verb.

Because after a verb like dicit, Latin often uses an indirect statement construction, also called the accusative and infinitive construction.

In that pattern:

• the subject of the reported statement goes into the accusative
• the verb of the reported statement goes into the infinitive

So in this sentence:

• rosas et lilia are the things said to be growing
• nasci is the infinitive

Even though roses and lilies would be the subject in English, Latin puts them in the accusative here because they are the subject inside the indirect statement.

Because lilium is a neuter noun.

Neuter nouns have a special rule:

• nominative plural ends in -a
• accusative plural also ends in -a

So:

• singular: lilium
• plural nominative/accusative: lilia

That is why lilia is correct here.

By contrast, rosa is feminine, so its accusative plural is rosas.

Nasci is the present infinitive of nascor.

A few important things about it:

• nascor is a deponent verb
• deponent verbs look passive in form
• but they have an active meaning

So although nasci looks like a passive infinitive, it means something active like:

• to be born
• to arise
• to grow / spring up in the case of plants

Because Latin is using indirect statement after dicit.

Compare:

• direct statement: Rosae et lilia nascuntur = Roses and lilies grow
• indirect statement after dicit: dicit rosas et lilia nasci = she says that roses and lilies grow

So Latin does not usually say dicit quod... here in basic prose. Instead it prefers:

• accusative for the subject of the reported statement
• infinitive for the reported verb

That is why nasci appears.

Because the preposition ex takes the ablative.

So:

• ex = out of, from
• terra must therefore be ablative singular
• bona also has to be ablative singular feminine to agree with terra

With macrons, this would be:

• ex bonā terrā

So the grammar is:

• ex
  • ablative
• bonā terrā = from good soil / out of good earth

Because bona must agree with terra in:

• gender: feminine
• number: singular
• case: ablative

Since terra is ablative singular after ex, the adjective must match it:

• terra → ablative singular feminine
• bona → ablative singular feminine

So the phrase is:

• ex bona terra

If macrons are written, you would see:

• ex bonā terrā

Both ideas are related, and the exact English wording depends on context.

With people or animals, nascor / nasci often means:

• to be born

With plants, it commonly means:

• to spring up
• to grow
• to arise from

So in this sentence, with rosas et lilia and ex bona terra, the natural meaning is about plants growing or springing from the soil.

It is normal Latin word order, but Latin is more flexible than English.

The sentence is arranged roughly like this:

• Avia dicit — main clause
• rosas et lilia — accusative subject of the indirect statement
• ex bona terra — prepositional phrase
• nasci — infinitive, often placed near the end

Latin often puts the infinitive or main verb of a clause toward the end, so nasci at the end feels natural.

You could rearrange the words and still keep the same basic meaning, as long as the forms stay clear.

Dicit is present, and nasci is a present infinitive.

In indirect statement, the present infinitive usually shows action that is simultaneous with the main verb.

So the sense is that, at the time grandmother speaks, the roses and lilies are growing or grow.

Very roughly:

• dicit ... nasci = she says that they grow / are growing

If Latin wanted to show an earlier action, it would use a different infinitive form.