Puer canem vocat, sed is non venit.

Because canem is the accusative singular form, used for the direct object of the verb.

• canis = the dog, in its basic dictionary form
• canem = the dog, when it is the thing being called

In this sentence, the boy is doing the action, and the dog is receiving the action:

• Puer = subject
• canem = direct object

So Latin uses canem, not canis.

Because puer is the subject of vocat.

Latin marks the subject with the nominative case, and puer is already nominative singular. So it does not need to change here.

You can think of it like this:

• puer = the boy, as the doer of the action
• canem = the dog, as the receiver of the action

English mostly shows this with word order, but Latin often shows it with word endings.

Is means he, she, or it, depending on context. In this sentence it means he or it, referring back to the dog.

Latin often uses is, ea, id as a basic third-person pronoun:

• is = he / it
• ea = she / it
• id = it

Here is is masculine singular, so it points back to a masculine noun or a noun being treated as masculine.

In this sentence, it most naturally refers to canem.

So the sense is:

• The boy calls the dog, but he/it does not come.

Grammatically, is could sometimes be ambiguous in another sentence, but here the meaning and context strongly point to the dog as the one not coming.

Yes. Latin often omits subject pronouns when the verb already makes the person and number clear.

Since venit already means he/she/it comes, Latin could simply say:

• Puer canem vocat, sed non venit.

Adding is makes the subject a little more explicit, almost like:

• but he does not come
• but that one does not come

So is is not required, but it is perfectly normal.

Because both verbs are third person singular in the present tense.

• vocat = he/she/it calls
• venit = he/she/it comes

The -t ending is a very common sign of third person singular in Latin verbs.

So:

• puer canem vocat = the boy calls the dog
• is non venit = he/it does not come

Even though the verbs come from different conjugations, they still show the same person and number here.

Because non is the usual Latin word for not, and it normally goes directly before the word it negates.

So:

• non venit = does not come

This is very straightforward and works much like English not, though Latin word order is generally more flexible.

Latin word order is much more flexible than English because Latin uses cases to show what each noun is doing.

So all of these could be understood:

• Puer canem vocat
• Puer vocat canem
• Canem puer vocat

The endings tell you that:

• puer is the subject
• canem is the object

That said, word order can still affect emphasis or style. The version in your sentence is a very normal, simple order.

They are both present tense.

• vocat = calls / is calling
• venit = comes / is coming

Latin present tense can often be translated in more than one way in English, depending on context. So venit could mean:

• comes
• is coming

But in a simple sentence like this, comes is the most natural translation.

Sed means but.

It connects the two parts of the sentence and shows a contrast:

• The boy calls the dog
• but the dog does not come

So sed signals that the second part goes against what you might expect from the first part.

Canis can be masculine or feminine, depending on the actual dog.

In this sentence, the pronoun is is masculine, so the dog is being referred to as masculine here, or at least treated that way grammatically.

So a learner should remember:

• canis is not fixed to only one gender in all situations
• the pronoun used later can help show how the noun is being treated in that sentence

A good way to break it down is:

• Puer = subject
• canem = direct object
• vocat = verb
• sed = conjunction
• is = pronoun referring back to the dog
• non venit = negative verb phrase

So the structure is:

• [Subject] [Object] [Verb], but [Pronoun] [not + Verb]

This is a useful beginner pattern because it shows several important Latin features at once:

• nominative subject
• accusative object
• present tense verbs
• a pronoun referring back to an earlier noun
• negation with non