Infans canem placidum tangit.

Because infans is in the nominative singular, the case normally used for the subject.

In Infans canem placidum tangit, infans is the one doing the action, and tangit is a singular verb, so they fit together: the child touches.

Because canem is the accusative singular form of canis.

Latin uses the accusative case for the direct object, the person or thing directly affected by the verb. Since the dog is what the child is touching, Latin puts canis into the accusative: canem.

So:

• canis = a dog, the dog, as subject
• canem = a dog, the dog, as direct object

Because placidum is an adjective describing canem, and in Latin adjectives must agree with the nouns they describe in case, number, and gender.

Here canem is:

• accusative
• singular
• masculine

So the adjective must also be:

• accusative
• singular
• masculine

That gives placidum.

We know because of agreement.

Placidum is accusative singular masculine, which matches canem. It does not match infans, which is nominative singular.

So the grammar tells you that the adjective goes with canem: the calm dog, not the calm child.

Tangit is the third-person singular present active indicative of tangere, meaning to touch.

That means it translates as:

• he touches
• she touches
• it touches

Since the subject is infans, the sentence means the child touches.

The ending -it is the clue that the verb is third-person singular.

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

So infans can mean:

• a child
• the child

and canem placidum can mean:

• a calm dog
• the calm dog

Which one sounds best depends on the context.

Less important than it is in English. Latin relies heavily on endings, so the forms of the words show their roles in the sentence.

Here:

• infans = subject
• canem placidum = object
• tangit = verb

Because of that, Latin can often change the word order without changing the basic meaning. For example:

• Infans canem placidum tangit
• Canem placidum infans tangit
• Tangit infans canem placidum

All can mean essentially the same thing, though the emphasis may shift.

Both are third-declension nouns.

• infans, infantis = child
• canis, canis = dog

That is why their forms do not look like first- or second-declension nouns such as puella or servus.

For this sentence, the important forms are:

• infans = nominative singular
• canem = accusative singular

No. Infans can refer to a male or female child.

So English might translate it as:

• the child
• the infant
• sometimes the boy or the girl, if the context makes that clear

The noun itself does not force one sex or the other here.

Yes. Since placidum is masculine accusative singular, it shows that canem is being treated as masculine here.

That is useful because canis itself can be masculine or feminine depending on context. The adjective makes the intended gender clear in this sentence.

If the dog were feminine, the adjective would be placidam instead.