Tristitia puerum flere cogit.

Because puerum is in the accusative case, which marks the direct object.

In this sentence, tristitia is doing the causing, and the boy is the one being made to do something. So the boy is the object of cogit.

• puer = nominative singular, the boy as subject
• puerum = accusative singular, the boy as object

So Latin uses puerum because the boy is not the subject of cogit.

Tristitia is in the nominative singular, which is the case normally used for the subject.

The verb cogit is third person singular, so it needs a singular subject, and tristitia fits that role.

So the basic structure is:

• tristitia = subject
• cogit = verb
• puerum flere = the boy to cry

Even though sadness is an abstract idea, Latin can use abstract nouns as subjects very naturally, just as English can in sentences like Fear drives him away or Anger made her shout.

Because after cogit, Latin commonly uses an accusative + infinitive construction.

Here:

• puerum = the person compelled
• flere = the action he is compelled to do

So cogit means forces / compels / makes, and the pattern is:

someone/something + accusative person + infinitive

Examples of the same pattern:

• magister puerum laborare cogit = the teacher makes the boy work
• timor milites fugere cogit = fear makes the soldiers flee

So flere is infinitive because it is not the main verb of the sentence; it depends on cogit.

In a sense, yes. The Latin infinitive flere already means to cry.

Latin does not need a separate word for to before an infinitive. The infinitive ending itself carries that idea.

So:

• flere = to cry
• not to + another word
• just one word in Latin

That is why Latin says puerum flere cogit, not something with a separate word for to.

Cogo often takes:

• a direct object in the accusative
• plus an infinitive

So the pattern is:

subject + accusative person/thing + infinitive + cogit

In this sentence:

• tristitia = what does the compelling
• puerum = the person being compelled
• flere = what he is compelled to do
• cogit = makes/forces

A very literal way to understand it is:

Sadness compels the boy to cry.

This is a very important Latin pattern, so it is worth getting used to early.

They are:

• tristitia — nominative singular
• puerum — accusative singular
• flere — present active infinitive (not a case form, since it is a verb)

And the final word:

• cogit — third person singular present active indicative

So each word has a clear job:

• tristitia = subject
• puerum = object of cogit
• flere = complementary infinitive
• cogit = main verb

Yes. Latin word order is more flexible than English word order because the endings show each word’s function.

So these could all mean essentially the same thing:

• Tristitia puerum flere cogit.
• Puerum tristitia flere cogit.
• Tristitia cogit puerum flere.
• Puerum flere tristitia cogit.

However, changing the order can change the emphasis.

For example:

• putting puerum first may emphasize the boy
• putting flere earlier may emphasize crying
• keeping cogit at the end is very common Latin style

So the meaning stays basically the same, but the focus can shift.

Because its subject, tristitia, is singular.

Cogit means he/she/it compels or makes. Since tristitia is a singular noun, the verb must also be singular.

If the subject were plural, the verb would be plural too. For example:

• tristitia cogit = sadness compels
• tristitiae cogunt = sorrows compel or sad things compel, depending on context

So the ending -t tells you the verb is third person singular.

The dictionary form is cogo, and its principal parts are:

cogo, cogere, coegi, coactum

From these you can identify:

• cogo = I compel, force
• cogere = to compel
• cogit = he/she/it compels

This is a third-conjugation verb.

A learner may notice that cogo looks a little unusual because it comes historically from co-ago, but for normal learning purposes it is best just to memorize it as:

cogo, cogere, coegi, coactum

The dictionary form is fleo, usually given as:

fleo, flere, flevi, fletum

Here flere is the present active infinitive, meaning to cry or to weep.

So:

• fleo = I cry
• flet = he/she/it cries
• flere = to cry

This is a second-conjugation verb.

It can mean sadness, but often also sorrow, grief, or gloom, depending on context.

So in a sentence like this, the idea may be more than mild unhappiness. It can suggest an emotion strong enough to drive someone to tears.

That is why cogit works well with it: the sentence expresses a strong emotional cause.

Yes, it is helpful to think of puerum flere as a grammatical unit after cogit.

Together they mean something like:

the boy to cry

This is not a complete English sentence, but it is a normal Latin structure. Latin often expresses ideas in this compact way, where English might use:

• makes the boy cry
• forces the boy to cry

So when reading, it helps to group the words like this:

tristitia | puerum flere | cogit

That makes the structure much easier to see.

Yes. Even if the meaning is already known, it is useful to notice that Latin often allows several good English renderings.

Possible translations include:

• Sadness makes the boy cry.
• Sadness forces the boy to cry.
• Sadness compels the boy to weep.
• Grief makes the boy weep.

The exact English choice depends on how strongly you want to render tristitia and cogit.

Latin often leaves room for a range of natural English translations rather than just one perfect wording.