In valle parva herba alta est, sed in monte saepe paucae arbores sunt.

Because in with the ablative usually means in or on in the sense of location.

• in valle = in the valley
• in monte = on/in the mountain depending on context, though in English we would usually say on the mountain

If in were followed by the accusative, it would usually mean into or onto, showing motion toward something.

Vallis is the dictionary form, the nominative singular. But after in showing location, Latin uses the ablative singular, so vallis changes to valle.

So:

• nominative: vallis = valley
• ablative: valle = in/on/from the valley, depending on the preposition or use

This is a normal third-declension pattern.

Because parva is an adjective modifying valle, and Latin adjectives must agree with the noun they describe in:

• gender
• number
• case

Here, valle is:

• feminine
• singular
• ablative

So the adjective must also be feminine singular ablative: parva.

Together, in valle parva means in the small valley.

Herba is the subject: grass.
Alta is a predicate adjective: tall.
Est means is.

So literally, this is:

• herba = grass
• alta = tall
• est = is

Latin often uses this pattern: noun + adjective + est.

Also, alta agrees with herba, which is:

• feminine
• singular
• nominative

So alta is also feminine singular nominative.

Because it describes herba, and herba is a feminine noun.

In Latin, adjectives must match the noun they describe. Since herba is feminine singular nominative, the adjective must also be feminine singular nominative:

• herba alta = tall grass

Even though English adjectives do not change form, Latin adjectives do.

Because arbor is a feminine noun.

So when paucus, -a, -um means few, it must agree with arbores:

• arbores = feminine plural nominative
• therefore paucae = feminine plural nominative

So:

• paucae arbores = few trees

If the noun were masculine plural, then you would use pauci.

Because the verbs agree with their subjects in number.

• herba is singular, so Latin uses est = is
• arbores is plural, so Latin uses sunt = are

So:

• herba alta est = the grass is tall
• paucae arbores sunt = there are few trees or literally few trees are

Latin often does not need a special word for there in this kind of sentence.

English says:

• there are few trees

Latin simply says:

• few trees are = paucae arbores sunt

This is a very normal Latin way to express existence. The idea of there are is understood from the context.

Saepe is an adverb meaning often.

It modifies the idea expressed by sunt in the second clause. So the sense is that often there are few trees on the mountain.

Latin adverb placement is flexible, but saepe is commonly placed near the word or phrase it helps emphasize.

Latin word order is much more flexible than English word order because Latin shows grammatical relationships mainly through endings, not position.

In English, word order is crucial:

• the grass is tall

In Latin, endings already show which word is the subject and which adjective goes with which noun, so the order can vary.

This sentence uses a very natural Latin arrangement:

• In valle parva herba alta est
• sed in monte saepe paucae arbores sunt

A different order could still mean the same thing, though it might change emphasis.

Latin has no articles. It does not have separate words for the or a/an.

So:

• herba can mean grass, the grass, or a grass/plant, depending on context
• monte can mean the mountain or a mountain
• arbores can mean trees or the trees

The correct English article has to be supplied from the meaning and context.

Grammatically, in + ablative means location, so in monte literally means in/on the mountain.

But when translating into natural English, we usually say:

• on the mountain

English chooses the most natural expression, while Latin simply uses in with the ablative to show location.

Sed means but.

It joins the two clauses and shows a contrast:

• in the small valley, the grass is tall
• but on the mountain, there are often few trees

So sed is a very common coordinating conjunction used to contrast one statement with another.