Sagitta e arcu volat, sed puella dicit se hastam graviorem esse putare quam telum leve.

Because e/ex means out of or from, and that is the normal preposition for something coming from a bow.

So:

• e arcu = from/out of the bow
• sagitta e arcu volat = the arrow flies from the bow

A native English speaker might expect several different Latin prepositions for from, but Latin chooses them by idea:

• e/ex = out of, from within
• ab = away from, from beside
• de = down from, about/concerning, from off

For an arrow shot from a bow, e/ex is the natural choice.

Because arcus is a fourth-declension noun, not a second-declension one.

Its ablative singular is arcu, so after e/ex you get:

• e arcu

A learner might expect -o because many Latin nouns use -o in the ablative singular, but fourth-declension masculine nouns usually use -u instead:

• servus → servo (2nd declension)
• arcus → arcu (4th declension)

So e arcu is exactly what you should expect from arcus.

Because Latin word order is much freer than English word order.

English usually needs something like:

• The arrow flies from the bow.

Latin can say:

• Sagitta e arcu volat

Putting the verb last is very common in Latin, especially in simple statements. It does not change the basic meaning. It just reflects normal Latin style.

Because Latin usually expresses reported statements with an accusative-and-infinitive construction, often called indirect statement.

English says:

• The girl says that she thinks ...

Latin often says the equivalent of:

• The girl says herself to think ...

That sounds strange in English, but it is standard Latin grammar.

So here:

• puella dicit = the girl says
• se ... putare = that she thinks ...

Latin often does this after verbs like:

• dicit = says
• putat = thinks
• scit = knows
• audit = hears

Se is the reflexive pronoun, and here it is the subject of the infinitive putare.

That is an important idea: in Latin indirect statement, the subject of the infinitive appears in the accusative.

So:

• puella dicit se putare = the girl says that she thinks

The se refers back to puella.

It does not mean that the girl is acting on herself. It is not the direct object of dicit in the ordinary English sense. Instead, it is the accusative subject inside the reported statement.

Because the sentence contains one reported statement inside another.

Structure:

• puella dicit
  • se ... putare
    • hastam graviorem esse quam telum leve

So the layers are:

1. dicit introduces an indirect statement:

   • se putare = that she thinks

2. putare itself introduces another indirect statement:

   • hastam graviorem esse = that the spear is heavier

So the whole thing is roughly:

• The girl says [that she thinks [that the spear is heavier than a light weapon/projectile]].

Latin handles both layers with the accusative-and-infinitive pattern.

Because hasta is the subject of esse inside an indirect statement, so it must be in the accusative:

• hasta → hastam

And graviorem agrees with hastam in:

• gender: feminine
• number: singular
• case: accusative

So:

• hastam graviorem esse = that the spear is heavier

If this were a direct, non-reported statement, you would expect:

• hasta gravior est

But inside indirect statement, the nominative becomes accusative:

• hasta gravior est
  → hastam graviorem esse

For the same reason: graviorem must agree with hastam, which is accusative singular feminine.

The adjective gravior means heavier, but its form changes by case:

• nominative feminine singular: gravior
• accusative feminine singular: graviorem

Since the phrase is:

• hastam graviorem esse

the comparative adjective has to match the accusative subject hastam.

Because telum is neuter, and the adjective must agree with it.

The adjective levis, leve has:

• masculine/feminine accusative singular: levem
• neuter accusative singular: leve

Since telum is neuter, the correct form is:

• telum leve

Also, in the neuter singular, nominative and accusative are the same in form, so telum leve can look identical in both cases.

Quam is used after a comparative adjective to mean than.

So:

• gravior = heavier
• graviorem ... quam telum leve = heavier than a light weapon/projectile

A useful rule is that with quam, the thing compared is usually in the same case as the thing it is being compared with.

Here:

• hastam is accusative
• telum leve is also accusative in function

So the comparison is balanced.

English learners sometimes want to supply extra words, but Latin often leaves them out because they are understood. The idea is simply:

• that the spear is heavier than a light weapon/projectile

It is the subject of putare, not of dicit.

The subject of dicit is already clear from puella:

• puella dicit = the girl says

Then the reported statement begins:

• se putare = that she thinks

So the sentence has two different subject levels:

• main clause subject: puella
• infinitive clause subject: se (referring back to puella)

That distinction is very common in Latin reported speech.

Yes. That would be a simpler sentence:

• puella putat hastam graviorem esse quam telum leve
  = the girl thinks that the spear is heavier than a light weapon/projectile

But your sentence says more specifically:

• puella dicit se ... putare
  = the girl says that she thinks ...

So dicit adds another layer: we are not just told what she thinks; we are told that she says this.

Because the verb ending already tells you the person and number, and the noun puella already names the subject.

So Latin does not need an extra word for she:

• puella dicit already means the girl says
• sagitta volat already means the arrow flies

Latin often leaves out subject pronouns unless they are needed for emphasis or contrast.

Your expected order is very reasonable, and it is also good Latin:

• se putare hastam graviorem esse

But Latin word order is flexible, and writers often move words around for rhythm, emphasis, or style.

So:

• se hastam graviorem esse putare

still makes sense, because the endings show how the words fit together:

• se goes with putare
• hastam graviorem esse is the statement being thought

A learner should focus first on endings and structure, not on forcing Latin into English word order.

A helpful way to understand it is to turn it back into direct speech.

Indirect:

• puella dicit se hastam graviorem esse putare quam telum leve

Direct speech would be something like:

• Puella dicit: hastam graviorem esse quam telum leve puto.

More naturally ordered Latin:

• Puella dicit: puto hastam graviorem esse quam telum leve.

So the indirect statement works by changing:

• ego / implied I → se
• finite verb like puto → infinitive putare

That is one of the most useful ways to learn Latin indirect statement.