Fagus in monte crescit, sed fraxinus et ulmus prope agrum stant.

Because the two location words depend on different prepositions.

• in
  • ablative usually means in/on/at a place with no motion toward it, so in monte means on the mountain or in the mountain area.
• prope takes the accusative, so prope agrum means near the field.

This is a very common Latin pattern:

• in urbe = in the city
• in urbem = into the city
• prope viam = near the road

So here monte is ablative because of in, and agrum is accusative because of prope.

With in, the case depends on whether there is motion.

• in
  • ablative = location, being somewhere
• in
  • accusative = motion into/toward somewhere

In this sentence, the beech tree is already located on the mountain; it is not moving there. So Latin uses in monte, not in montem.

The verb agrees with its subject.

• Fagus is one tree, so the verb is singular: crescit = grows
• fraxinus et ulmus are two trees joined by et, so the verb is plural: stant = stand

So Latin works much like English here:

• one subject → singular verb
• more than one subject → plural verb

Classical Latin has no articles. It does not normally use separate words for the, a, or an.

So:

• fagus can mean a beech tree or the beech tree
• agrum can mean a field or the field

You decide from the context which English article is most natural.

They are nominative, because they are the subjects of the verbs.

• fagus is the subject of crescit
• fraxinus et ulmus are the subjects of stant

In Latin, the nominative case is usually used for the subject of a finite verb.

The basic forms are:

• fagus, fagi = beech tree
• mons, montis = mountain
• fraxinus, fraxini = ash tree
• ulmus, ulmi = elm tree
• ager, agri = field

And the forms in the sentence are:

• fagus = nominative singular
• monte = ablative singular of mons
• fraxinus = nominative singular
• ulmus = nominative singular
• agrum = accusative singular of ager

Latin can describe plants in more than one natural way.

• crescere means to grow
• stare means to stand

So:

• Fagus in monte crescit emphasizes the beech as something growing there.
• fraxinus et ulmus prope agrum stant describes the ash and elm as standing there.

This is not a contradiction. English also varies between grows and stands when describing plants or trees.

Latin has much freer word order than English because the endings show the grammatical roles.

English depends heavily on order:

• subject + verb + object

Latin can move words around more easily for style, emphasis, or rhythm. In this sentence, the order is quite natural and clear:

• Fagus in monte crescit
• sed fraxinus et ulmus prope agrum stant

A very literal rearrangement into English order might be:

• The beech grows on the mountain, but the ash and the elm stand near the field

But Latin does not need to keep exactly that order to remain understandable.

Sed means but and introduces a contrast.

The sentence is setting one statement against another:

• the beech is on the mountain
• but the ash and elm are near the field

So sed marks the contrast in location.

Et simply means and. It joins the two nouns into a compound subject:

• fraxinus et ulmus = the ash and the elm

Because they are joined together, they take the plural verb stant.

Yes. Tree names in Latin are often feminine, even when their form may look masculine to a beginner.

In this sentence:

• fagus is feminine
• fraxinus is feminine
• ulmus is feminine

This can surprise English speakers, especially because nouns ending in -us are very often masculine. Tree names are one of the important exceptions worth learning.

The main grammar point is that the prepositions control the cases.

For example:

• in monte = on the mountain
• prope agrum = near the field

If you changed the prepositions, the forms might change too. For instance:

• sub monte = under the mountain, at the foot of the mountain
• ad agrum = toward the field

So one thing to notice in this sentence is not just the vocabulary, but how each preposition determines the case of the noun after it.

A simple classroom pronunciation would be roughly:

• fagus = FAH-goos
• in monte = in MON-teh
• crescit = KRES-kit
• sed = sed
• fraxinus = FRAK-see-noos
• et ulmus = et OOL-moos
• prope agrum = PRO-peh AH-groom
• stant = stahnt

If you are learning a specific pronunciation system, such as restored Classical or ecclesiastical, your teacher may want slightly different sounds. The grammar of the sentence stays the same either way.