Medicus tergum pueri videt.

Because medicus is in the nominative singular, which is the case usually used for the subject of a sentence.

Also, the verb videt means he/she/it sees, so it needs a singular subject. Medicus fits that perfectly: the doctor.

So the basic structure is:

• medicus = the doctor = subject
• videt = sees = verb

Tergum is in the accusative singular, which is the case commonly used for the direct object.

In this sentence, the thing being seen is the back, so tergum is the object of videt:

• medicus = the doctor
• tergum = the back
• videt = sees

So the doctor sees the back ...

Then pueri adds whose back it is.

Because here Latin wants of the boy, not the boy as a direct object.

Pueri is the genitive singular form of puer, and the genitive often shows possession or a close relationship:

• puer = boy
• pueri = of the boy

So:

• tergum pueri = the boy’s back or the back of the boy

If it were puerum, that would be the boy as a direct object, which would give a different structure.

Yes, pueri can have more than one meaning depending on context:

• genitive singular = of the boy
• nominative plural = boys

But in this sentence, of the boy is the correct reading.

Why?

• medicus is already the subject.
• videt is singular, so a nominative plural pueri would not fit well as another subject.
• tergum pueri makes very good sense as the boy’s back.

So this is a good example of how Latin forms can sometimes be ambiguous by themselves, but the whole sentence makes the meaning clear.

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

So:

• medicus can mean doctor, a doctor, or the doctor
• tergum can mean back, a back, or the back

You decide which English wording sounds best from the context.

That is why one Latin sentence can often be translated into English in slightly different ways, such as:

• The doctor sees the boy’s back
• A doctor sees the back of the boy

It matters somewhat, but less than in English.

Latin relies heavily on case endings, so the endings tell you what each word is doing. Because of that, the sentence could be rearranged and still mean basically the same thing:

• Medicus tergum pueri videt
• Tergum pueri medicus videt
• Pueri tergum medicus videt

All of these can still mean The doctor sees the boy’s back, because:

• medicus is nominative
• tergum is accusative
• pueri is genitive
• videt is the verb

That said, word order can still affect emphasis. The version you have is a very straightforward order.

Videt comes from the verb video, videre, meaning to see.

The form videt is:

• present tense
• active voice
• indicative mood
• third person singular

So it means:

• he sees
• she sees
• it sees

In this sentence, it means the doctor sees.

Latin often puts the verb near or at the end of the sentence, especially in simple textbook sentences.

That is a very common Latin pattern, though not a strict rule.

So this sentence follows a very familiar arrangement:

• subject: medicus
• object phrase: tergum pueri
• verb: videt

English usually prefers The doctor sees the boy’s back, with the verb earlier, but Latin often saves the verb until the end.

All three nouns here belong to the second declension, but not all in exactly the same way.

• medicus: second declension, masculine
• tergum: second declension, neuter
• puer: second declension, masculine

A few useful forms:

• medicus → nominative singular
• tergum → nominative/accusative singular neuter
• pueri → genitive singular of puer

A helpful thing to remember is that neuter second-declension nouns have the same form in the nominative and accusative singular, so tergum can be either nominative or accusative by form alone. Here it is accusative because it is the object.

The dictionary forms are usually:

• medicus, medici = doctor
• tergum, tergi = back
• puer, pueri = boy
• video, videre, vidi, visum = see

For a beginner, it is especially useful to notice that puer keeps the -e- in forms like pueri. Not every second-declension masculine noun ends in -us in the nominative singular.

Not only that.

Tergum basically means back, especially the back part or rear surface of something. Depending on context, it can refer to:

• a person’s back
• an animal’s back
• the back side of an object

In this sentence, because of pueri, it clearly means the boy’s back.

A simple classroom pronunciation would be something like:

MEH-di-koos TER-goom POO-eh-ree WEE-det

A few helpful notes:

• c in classical Latin is always hard, like k
• v in reconstructed classical pronunciation is like English w
• g is always hard, as in go
• ae and other vowel combinations matter in some words, though not in this sentence

So in a more classical-style pronunciation, videt may sound closer to wee-det than English vee-det.

Yes. Both are good English translations.

The genitive pueri can be expressed in English in either of these ways:

• the back of the boy
• the boy’s back

English usually prefers the boy’s back, because it sounds more natural, but both accurately reflect the Latin structure.