Post pluviam campus tam viridis est ut avus diu eum spectet.

Because post takes the accusative case when it means after in the sense of time.

So:

• post pluviam = after the rain

This is just a normal prepositional pattern you have to learn:

• post bellum = after the war
• post cenam = after dinner
• post noctem = after nightfall

So pluviam is accusative not because it is a direct object, but because post requires it.

Latin has no articles like English the or a/an.

That means a noun like campus can mean:

• the field
• a field

and pluviam can mean:

• the rain
• rain
• sometimes even a rainstorm, depending on context

The context tells you which English article makes the most sense.

tam ... ut is a very common Latin pattern meaning so ... that.

In this sentence:

• tam viridis est = is so green
• ut avus diu eum spectet = that grandfather watches it for a long time

So tam sets up a degree, and ut introduces the result that follows from that degree.

Very often, when you see tam, ita, sic, tot, or tantus, you should be ready for an ut clause afterward.

Examples:

• tam fessus est ut dormiat = he is so tired that he sleeps
• tot libri sunt ut legere non possim = there are so many books that I cannot read them

Because the clause introduced by ut after tam is a result clause, and result clauses normally use the subjunctive in Latin.

So:

• tam viridis est = the main statement
• ut ... spectet = the result of that greenness

This is one of the standard uses of the subjunctive that English speakers have to get used to. In English, we usually keep the verb in the ordinary form:

• so green that grandfather watches it

But Latin prefers the subjunctive in this kind of clause:

• ut avus diu eum spectet

The big clue is tam.

A clause with tam ... ut strongly points to result:

• so green that ...

A purpose clause usually answers a question like for what purpose? or in order that what might happen? A result clause answers with what result?

Here the structure is:

• the field is so green
• that grandfather looks at it for a long time

That is clearly a result, not a purpose.

A useful rule:

• ut by itself can be purpose or result
• words like tam, ita, sic, adeo, tantus, tot often signal result

Because the main verb is est (present), and Latin normally uses the present subjunctive in a subordinate clause when the action is viewed as happening at the same time from a present main verb.

So:

• campus ... est = the field is
• ut ... spectet = so that grandfather looks / may look / would look at it

The exact English wording can vary, but grammatically the Latin is treating the looking as contemporaneous with the field’s being green.

If the main verb were in a past tense, you would often expect an imperfect subjunctive in the subordinate clause:

• campus tam viridis erat ut avus diu eum spectaret

The subject is avus.

You can tell because:

• avus is nominative singular
• eum is accusative singular, so it is the object
• spectet is third person singular, matching avus

So the structure is:

• avus = grandfather, subject
• eum = him/it, direct object
• spectet = looks at / watches

eum refers back to campus.

Why?

• campus is masculine singular
• eum is masculine singular accusative
• it makes sense in context: grandfather is looking at the field

So:

• campus = the field
• eum = it (literally masculine him, but in English we say it for a field)

Latin personal pronouns often agree with the noun they refer to in gender and number, while their case depends on their role in the sentence. Here it is accusative because it is the object of spectet.

Yes, Latin could often leave it out if the object were obvious from context. But using eum makes the reference explicit: grandfather is looking at it, namely the field.

Latin often omits subject pronouns, but object pronouns may still be expressed when the writer wants:

• clarity
• emphasis
• smoother style

So eum is not strange here; it simply makes the object clear.

Because grammatical gender in Latin is not the same as natural gender.

campus is simply a masculine noun by grammatical classification. That is why adjectives and pronouns referring to it are masculine too:

• campus = masculine
• viridis agrees with it
• eum refers back to it in masculine form

English speakers often expect gender to match real-world sex, but in Latin most nouns referring to things are assigned grammatical gender that you just have to learn with the word.

Because viridis agrees with campus.

campus is:

• nominative
• singular
• masculine

And viridis is a third-declension adjective whose masculine and feminine nominative singular form is viridis.

So campus viridis means a green field / the field is green.

If the noun were neuter singular, the adjective would be viride:

• pratum viride = a green meadow

diu is an adverb meaning for a long time or long.

It modifies spectet:

• avus diu eum spectet = grandfather looks at it for a long time

It tells you how long the action continues.

Compare:

• multum can sometimes mean much / a lot
• diu specifically refers to duration of time

Because Latin word order is much more flexible than English word order.

English depends heavily on position:

• the grandfather sees the field
• the field sees the grandfather
  These mean different things because the order changes.

Latin relies much more on case endings:

• campus is nominative, so it is the subject
• eum is accusative, so it is the object

That means Latin can move words around for emphasis, rhythm, or style.

In this sentence, the order helps the reader feel the structure:

• Post pluviam sets the time first
• campus introduces the main subject
• tam viridis est sets up the degree
• ut ... gives the result

So the order is not random; it is stylistic and meaningful.

Yes, but with a caution.

In English, so that can express either:

• purpose: I came early so that I could help.
• result: It was so cold that the lake froze.

Here the Latin is definitely the second kind: result.

So translating ut as that is often the clearest choice in English:

• so green that ...

You can say so that if your English still sounds natural, but learners should remember that the grammar here is a result clause, not a purpose clause.

Yes, post pluviam is a perfectly normal compact way to say after the rain.

Latin can also express the idea with a clause, for example:

• postquam pluit = after it rains / after it has rained, depending on context
• more commonly in past narrative, forms with past tense would appear

The version in your sentence uses a noun phrase instead of a full clause:

• post pluviam = after the rain

That is simple, idiomatic Latin.

Avus specifically means grandfather.

If Latin wanted to say old man, it would usually use a different word, such as senex.

So here the sentence is definitely talking about a grandfather, not just any elderly man.

Yes. Spectare often means:

• to look at
• to watch
• to observe

It can suggest sustained looking, which fits well with diu.

So in this sentence, depending on context, English might use:

• look at
• gaze at
• watch

The grammar stays the same either way.