Puer ad saxum currit et de saxo in herbam salit.

Because puer is the subject of the sentence: the boy is the one doing the actions. Subjects are usually in the nominative case, and puer is nominative singular.

The other nouns are not subjects. They are used after prepositions, so their form depends on what the preposition requires:

• ad saxum = toward the rock → accusative
• de saxo = from the rock / down from the rock → ablative
• in herbam = into / onto the grass → accusative

So the endings change because Latin marks a noun’s job in the sentence with its case ending.

The preposition ad usually takes the accusative case and often means to, toward, or up to.

So:

• ad = toward
• saxum = accusative singular of saxum

That is why Latin says ad saxum, not ad saxus or ad saxo.

A useful rule is:

• ad + accusative = motion toward something

Because de takes the ablative case.

So:

• dictionary form: saxum = rock
• ablative singular: saxo

Thus:

• de saxo = from the rock / down from the rock

A basic rule is:

• de + ablative = down from, from, or sometimes about/concerning

Here it clearly shows movement away from the rock.

Because in can take two different cases, depending on the meaning.

• in + accusative = motion into or onto
• in + ablative = location in or on

Here the boy is jumping onto the grass, so Latin uses motion toward a place, which requires the accusative:

• in herbam = onto the grass

If the sentence meant that he was already on the grass, Latin would use:

• in herba = on the grass

This is one of the most important preposition patterns in Latin.

Saxum is a neuter second-declension noun.

That helps explain its forms:

• nominative singular: saxum
• accusative singular: saxum
• ablative singular: saxo

A very important neuter rule is:

• in the singular, neuter nominative and accusative are the same

So saxum can be both nominative and accusative singular. In this sentence it is accusative after ad.

Herba is a first-declension feminine noun.

Its basic forms include:

• nominative singular: herba
• accusative singular: herbam
• ablative singular: herba

So in in herbam, the ending -am shows that it is accusative singular, because the preposition in here expresses motion onto the grass.

Both are third-person singular present tense verbs.

That means each one means he/she/it does the action:

• currit = he runs
• salit = he jumps

In this sentence, both refer back to puer, so in English we translate them as:

• the boy runs
• and jumps

The ending -it here tells you the subject is third-person singular.

Classical Latin does not have definite and indefinite articles like English does.

So Latin does not normally have separate words for:

• the
• a/an

That means puer can mean:

• the boy
• a boy

and saxum can mean:

• the rock
• a rock

You decide which English article fits best from the context.

Yes. Latin word order is much freer than English word order because the case endings show how the words fit together.

So the sentence could be rearranged in various ways and still mean roughly the same thing, for example:

• Puer ad saxum currit et de saxo in herbam salit.
• Ad saxum puer currit et de saxo in herbam salit.
• Puer currit ad saxum et salit de saxo in herbam.

The original order is natural and clear, but Latin does not rely on word order as heavily as English does.

Et simply means and, and here it joins the two actions:

• currit
• salit

So the structure is:

• The boy runs ... and jumps ...

Latin often places et between the two words or phrases it connects, just like English.

In this sentence, de saxo is especially suitable because de often suggests movement down from the top or surface of something.

That fits well with jumping off a rock:

• de saxo = down from / off the rock

By contrast:

• ex often means out of something, especially from inside
• ab often means away from or from the side of

So de is the most natural choice when someone jumps off a rock.

Latin and English do not always treat nouns in exactly the same way.

Latin herba is a normal singular noun meaning grass, herb, or vegetation, depending on context. In this sentence, in herbam means something like:

• onto the grass
• into the grass

So even though English grass is often uncountable, Latin can still use the singular herbam very naturally here.