Si collum et dorsum sana sunt, puero iterum equitare licet.

Because it describes collum et dorsum together: the neck and the back.

• collum is a neuter singular noun.
• dorsum is also a neuter singular noun.
• When two neuter singular nouns are joined by et, Latin treats them as a neuter plural idea.

So:

• collum = healthy neck
• dorsum = healthy back
• collum et dorsum sana sunt = the neck and back are healthy

That is why the adjective is sana:

• nominative
• neuter
• plural

And the verb is also plural: sunt = are.

For the same reason: the subject is really two things joined together, collum et dorsum.

Even though each noun by itself is singular, together they make a plural subject, so Latin uses:

• sunt = are

not

• est = is

So:

• collum sanum est = the neck is healthy
• collum et dorsum sana sunt = the neck and back are healthy

Because of the verb licet.

Licet often works with:

• a dative person: the person to whom something is permitted
• an infinitive: the action that is permitted

So in this sentence:

• puero = to the boy / for the boy
• equitare = to ride
• puero equitare licet = the boy is allowed to ride / it is permitted for the boy to ride

This is a very common Latin pattern.

Because licet takes an infinitive to express the action that is allowed.

So:

• licet = it is permitted
• equitare = to ride

Together:

• equitare licet = it is permitted to ride

And with the dative:

• puero equitare licet = the boy is permitted to ride

This is similar to English to ride after is allowed.

Literally, licet means something like it is permitted or it is allowed.

So the Latin structure is a little more impersonal than natural English. Latin says:

• puero equitare licet

Literally:

• to the boy, it is permitted to ride

Natural English:

• the boy is allowed to ride
• the boy may ride

So licet is not really he allows or she allows. It is an impersonal verb meaning it is allowed / it is permitted.

Because this sentence is talking about permission, not just ability.

English sometimes uses can loosely, but Latin distinguishes ideas more clearly:

• possum = I am able / I can
• licet = it is permitted / one may

So if the meaning is the boy is allowed to ride again, licet is the right word.

The sentence is not mainly saying the boy is physically capable of riding; it is saying riding is now permitted, assuming the condition is met.

This is a straightforward, real condition:

• Si collum et dorsum sana sunt = If the neck and back are healthy
• puero iterum equitare licet = the boy may ride again

Both verbs are in the present indicative:

• sunt
• licet

This kind of si clause usually expresses a simple, open condition: if this is the case, then that is the result.

It does not suggest something imaginary or contrary to fact. It simply says: if the neck and back are healthy, then riding is permitted.

Iterum means again.

In this sentence:

• puero iterum equitare licet

it modifies the idea of riding:

• the boy is allowed to ride again

Latin word order is flexible, so iterum can often appear in different places without changing the basic meaning. Here it sits before equitare, which makes good sense: again to ride.

Latin uses case endings, so word order is freer than in English.

English depends heavily on order:

• the boy may ride again

Latin can show relationships through endings:

• puero is dative
• collum and dorsum are nominative
• sana agrees with them
• equitare is infinitive

Because of that, Latin does not need to keep the same fixed order as English. The sentence could not be translated word-for-word into natural English order without sounding odd.

The given order is perfectly normal Latin:

• condition first: Si collum et dorsum sana sunt
• main statement second: puero iterum equitare licet

They are in the nominative singular form, and together they make up the subject of sunt.

You can tell because:

• collum = nominative singular neuter
• dorsum = nominative singular neuter
• sana agrees with them as a combined neuter plural predicate adjective
• sunt is plural, matching the two-part subject

So:

• collum et dorsum = the neck and back
• sana sunt = are healthy

It is literally are healthy.

Latin often uses an adjective with a form of sum just like English does:

• sanus est = he is healthy
• sana sunt = they are healthy (for neuter plural things)

So nothing special is being omitted here. It is a normal linking-verb construction:

• subject: collum et dorsum
• adjective describing the subject: sana
• verb: sunt

Because after licet, Latin normally uses an infinitive to express the permitted action.

So Latin prefers:

• equitare licet = it is permitted to ride

rather than building the idea with a noun like riding.

This is just the normal idiom:

• alicui facere licet = someone is allowed to do something

Usually, yes.

Equitare comes from eques / equus and often means to ride horseback or to ride a horse.

Depending on context, English may translate it simply as ride, but in Latin the idea is typically horseback riding, not riding in a vehicle.

So the sentence most naturally suggests:

• the boy is allowed to ride horseback again

Yes. Both help explain the dative, though the best natural English translation is usually:

• the boy is allowed to ride again

If you are trying to understand the grammar literally, you can think:

• to the boy it is permitted to ride or
• for the boy it is permitted to ride

Both show why Latin uses the dative. But in normal English, we usually turn that into a subject:

• the boy may ride again
• the boy is allowed to ride again

Yes. Several translations are possible, depending on style:

• If the neck and back are healthy, the boy may ride again.
• If his neck and back are healthy, the boy is allowed to ride again.
• If the neck and back are sound, the boy may ride again.

The Latin grammar stays the same, but English gives some flexibility in how naturally or literally you want to translate it.