Post paucos dies semina in sulco iam crescunt, sed radices adhuc parvae sunt.

Because post is a preposition that takes the accusative and means after.

So:

• post = after
• paucos dies = a few days

The phrase literally means after a few days.

Here:

• paucos is accusative plural masculine
• dies is accusative plural

Because it agrees with dies.

The noun dies, diei is often masculine when it means an ordinary day or days, especially in the plural. So:

• pauci dies = a few days
• paucos dies = a few days (accusative after post)

Latin adjectives must agree with the noun they describe in:

• gender
• number
• case

Semina is nominative plural neuter from semen, seminis = seed.

Since semina is plural, the verb must also be plural:

• crescunt = they grow / are growing

So:

• semen crescit = the seed grows
• semina crescunt = the seeds grow

The ending -nt is the normal 3rd person plural ending in the present tense.

Because in can take either the ablative or the accusative, depending on the meaning.

• in + ablative = in / on a place, showing location
• in + accusative = into / onto a place, showing motion toward

Here the meaning is location: the seeds are growing in the furrow, not moving into it. So Latin uses:

• in sulco = in the furrow

If it meant motion, it would be:

• in sulcum = into the furrow

Because the number of sulco does not have to match the number of semina.

The sentence means the seeds are in the furrow or a furrow. Even though there are many seeds, they can still all be in one furrow. So singular sulco is perfectly normal.

They show time more precisely.

• iam = now already / by now
• adhuc = still / up to this point

So the contrast is:

• semina ... iam crescunt = the seeds are already growing
• radices adhuc parvae sunt = but the roots are still small

These two adverbs help show development: something has started happening, but another thing has not developed very far yet.

Because it agrees with radices.

Radix, radicis = root, and it is a feminine noun.
Radices is plural, so the adjective must also be feminine plural:

• radices parvae = small roots

Again, adjectives in Latin agree with the noun they describe in:

• gender
• number
• case

Because Latin usually does not need a separate subject pronoun.

The ending of the verb already tells you the subject:

• cresco = I grow
• crescis = you grow
• crescit = he/she/it grows
• crescunt = they grow

So crescunt already means they grow. Since semina is present, the subject is clear.

Latin word order is much freer than English word order.

All of these are grammatically possible:

• radices adhuc parvae sunt
• radices sunt adhuc parvae
• adhuc radices parvae sunt

The version in your sentence places sunt at the end, which is very common in Latin. It also keeps parvae close to radices, which makes the description clear.

Either can be correct, depending on context.

The Latin present tense often covers both:

• simple present: grow
• progressive present: are growing

So semina in sulco iam crescunt could be understood as:

• the seeds are already growing in the furrow or
• the seeds already grow in the furrow

In natural English, are growing is often the smoother translation here.

Sed means but and introduces a contrast.

The contrast is:

• the seeds are already growing
• but the roots are still small

So sed helps connect the two ideas and show that they do not fully match yet: growth has begun, but development is still limited.

Radices is nominative plural here, because it is the subject of sunt.

You can tell from the sentence structure:

• radices = the roots
• parvae sunt = are small

So radices is the thing being described, which makes it the subject.

From the noun radix, radicis:

• nominative singular: radix
• nominative plural: radices

Latin often puts a time expression near the beginning to set the scene.

So Post paucos dies tells you first when the rest happens:

• After a few days, the seeds are already growing...

This is a very common and natural Latin pattern: time expression + main statement.