Puer tandem fatetur se nummos in mensa reliquisse.

Because fatetur comes from fateor, which is a deponent verb.

Deponent verbs look passive in form, but they have an active meaning. So:

• fatetur = he admits / he confesses
• not he is admitted

This is very common in Latin, and fateor is one of the standard deponent verbs learners meet.

After verbs like fatetur (admits, confesses), Latin usually uses indirect statement, also called the accusative-and-infinitive construction.

Instead of saying:

• he admits that he left the coins on the table

Latin says, more literally:

• he admits himself to have left the coins on the table

That is why you get:

• se = the subject of the indirect statement
• reliquisse = the infinitive verb of the indirect statement

So se nummos in mensa reliquisse is the whole reported statement.

Se is the reflexive pronoun, used when the subject of the subordinate idea is the same as the subject of the main verb.

Here:

• Puer ... fatetur = The boy admits
• who left the coins? the boy himself

So Latin uses se = himself.

If the sentence meant that the boy admitted that someone else had left the coins, Latin would use a different pronoun, such as eum.

Reliquisse is the perfect active infinitive of relinquo.

In indirect statement, the tense of the infinitive shows time relative to the main verb:

• present infinitive = happening at the same time
• perfect infinitive = happening before the main verb
• future infinitive = happening after the main verb

So:

• fatetur = he admits
• reliquisse = to have left

This shows that the leaving happened before the admitting.

Puer is nominative singular, so it is the subject of fatetur.

You can tell from both:

1. its form: puer is the nominative singular of puer, pueri
2. the verb: fatetur is third person singular, matching puer

So the main clause is:

• Puer tandem fatetur = The boy finally admits

Nummos is the direct object of reliquisse.

The verb relinquo means leave behind, and the thing left is put in the accusative:

• nummos reliquisse = to have left the coins

Even though this is inside indirect statement, nummos still functions as the object of the infinitive.

Because in with the ablative usually means in/on a place, expressing location.

So:

• in mensa = on the table

But in with the accusative usually means into/onto a place, expressing motion toward.

So:

• in mensam would mean onto the table

Here the coins are already resting there, so Latin uses in mensa.

Tandem means something like:

• finally
• at last
• sometimes after all this time

It is an adverb, so its position is fairly flexible. Here it comes after puer, but Latin word order is much freer than English word order.

So Puer tandem fatetur means:

• The boy finally admits
• or At last the boy admits

The position of tandem helps give a natural emphasis, but it does not change the basic grammar.

The grammar does not depend mainly on word order, because Latin uses case endings and verb forms to show the relationships between words.

So this sentence can be understood even though the order is not the same as in English:

• Puer = subject
• fatetur = main verb
• se ... reliquisse = indirect statement
• nummos = object of reliquisse
• in mensa = location

The given order is perfectly natural, but Latin could rearrange parts of it for emphasis more easily than English can.

A learner might break the sentence down like this:

• puer → puer, pueri = boy
• tandem = finally, at last
• fatetur → fateor, fateri, fassus sum = admit, confess
  (a deponent verb)
• se = himself
• nummos → nummus, nummi = coin
• in = in, on
• mensa → mensa, mensae = table
• reliquisse → relinquo, relinquere, reliqui, relictum = leave behind

This kind of parsing is often the key to seeing how the sentence works.