Terra post pluviam bona est, et herba in campo alta est.

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

So:

• post = after
• pluviam = rain, in the accusative singular

The basic dictionary form is pluvia, but after post it becomes pluviam.

So post pluviam literally means after rain or after the rain.

Because bona has to agree with terra.

In Latin, adjectives agree with the nouns they describe in:

• gender
• number
• case

Here:

• terra is feminine singular nominative
• so the adjective must also be feminine singular nominative
• that gives bona

So:

• terra bona est = the الأرض/ground/land is good / the ground is good

If the noun were masculine, you might see bonus instead.

For the same reason as bona with terra: adjective agreement.

• herba is feminine singular nominative
• alta is the feminine singular nominative form of altus

So:

• herba alta est = the grass is tall

Latin adjectives must match the noun they describe.

Both are nominative singular because they are the subjects of their clauses.

• Terra ... bona est = terra is the subject
• herba ... alta est = herba is the subject

In Latin, the nominative case is usually used for the subject of the sentence.

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:

• in campo = in the field

So campo is ablative.

If the sentence meant movement, like into the field, Latin would use in campum.

Classical Latin does not have articles like English the or a/an.

So a noun like terra can mean, depending on context:

• earth
• land
• the land
• ground
• the ground

Likewise herba can mean:

• grass
• the grass
• an herb
• the herb

English usually has to choose an article, but Latin often leaves that to context.

Yes, Latin often can leave out forms of esse (to be) when the meaning is obvious, especially in some styles. But it is also very normal to include it.

So this sentence has two clear clauses:

• Terra post pluviam bona est
• et herba in campo alta est

Repeating est makes each clause complete and easy to follow.

A Latin author might sometimes omit the second est if the context made it obvious, but including both is perfectly normal and very clear for learners.

Latin word order is much more flexible than English word order because the endings show grammatical relationships.

So this sentence could be rearranged in various ways without changing the basic meaning, for example:

• Post pluviam terra bona est, et herba in campo alta est.
• Terra bona est post pluviam, et in campo herba alta est.

However, different word orders can change emphasis or style.

The given version is straightforward and natural for a learner:

• subject first
• then extra phrase
• then adjective + est

Terra has a range of meanings, including:

• earth
• land
• ground
• soil

In this sentence, because it is followed by post pluviam and described as bona, an English speaker will often understand it as something like:

• the ground is good after the rain
• the soil is good after the rain
• the land is good after the rain

The exact best translation depends on the context. Latin often allows a little more semantic range than a single English word does.

Et means and.

It connects the two clauses:

• Terra post pluviam bona est
• herba in campo alta est

So the whole sentence is simply joining two statements together.

Et is one of the most common Latin conjunctions.

A big clue is the dictionary form and the declension pattern.

• terra, terrae is a first-declension noun, usually feminine
• herba, herbae is also first declension, usually feminine
• campus, campi is a second-declension noun in -us, usually masculine

That is why:

• terra takes bona
• herba takes alta
• campo is from campus

There are exceptions in Latin, but these are very common patterns and good ones to learn early.