Sonus ingratus puerum terret.

The subject is sonus.

You can tell because sonus is in the nominative singular, which is the case normally used for the subject of a sentence. So sonus ingratus means the unpleasant sound and is the thing doing the action.

Because puerum is the direct object of the verb.

The verb terret means frightens or scares, so we need a word for the person being frightened. In Latin, a direct object usually goes in the accusative case, and the accusative singular of puer is puerum.

So:

• puer = boy, as subject
• puerum = boy, as direct object

Because ingratus is an adjective describing sonus, and Latin adjectives must agree with the nouns they describe.

That means they match in:

• gender
• number
• case

Here:

• sonus is masculine
• singular
• nominative

So the adjective is also:

• masculine
• singular
• nominative

That is why it appears as ingratus.

Not in this sentence.

Although ingratus can sometimes mean ungrateful, it can also mean unpleasant, disagreeable, or unwelcome. With sonus, it means something like an unpleasant sound or a disagreeable noise.

So the meaning here is about the sound being harsh or unwelcome, not about it having feelings.

Terret is:

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

It comes from the verb terreo, terrere, meaning to frighten or to scare.

The ending -t tells you it is he/she/it frightens. Since the subject is sonus, the translation is the unpleasant sound frightens the boy.

We know from the case endings, not mainly from word order.

• sonus is nominative, so it is the subject
• puerum is accusative, so it is the direct object

That means:

• sonus = the thing frightening
• puerum = the one being frightened

This is one of the biggest differences from English. In English, word order is very important. In Latin, the endings usually tell you the grammatical role.

Yes, it could come before.

Latin word order is much more flexible than English word order. So both of these are possible:

• sonus ingratus
• ingratus sonus

Both mean an unpleasant sound.

Sometimes the choice of order can give a slightly different emphasis, but both are grammatically normal.

Yes. Latin allows several possible word orders because the endings show the grammar.

For example, these would still mean basically the same thing:

• Sonus ingratus puerum terret
• Puerum sonus ingratus terret
• Terret puerum sonus ingratus

The most neutral English translation stays the same: The unpleasant sound frightens the boy.

Different orders in Latin often change emphasis, not the basic meaning.

Because Latin does not have articles like English the and a.

So sonus can mean:

• a sound
• the sound

And puerum can mean:

• a boy
• the boy

You decide from the context which English article sounds best. In this sentence, the unpleasant sound frightens the boy is a natural translation, but an unpleasant sound frightens a boy would also be possible in another context.

Sonus is a fourth-declension noun.

Its basic dictionary form is usually given as sonus, sonus, meaning the nominative singular and genitive singular are both sonus.

That can surprise learners, because the -us ending often makes people think of the second declension. But here it is fourth declension.

In this sentence, though, the important thing is simply that sonus is nominative singular, so it is the subject.

The basic dictionary form is puer, pueri.

That tells you it is a second-declension masculine noun. The form puerum is its accusative singular.

So when you look up puerum, you should recognize that it comes from puer and means boy.

A very literal translation would be:

Unpleasant sound frightens boy

To make it natural English, we usually add articles:

The unpleasant sound frightens the boy

or

An unpleasant sound frightens the boy

Latin often sounds more bare if translated word-for-word, because it does not use articles the way English does.