Avia per scalas lente ambulat, et puer eam ducit.

Because avia is in the nominative singular, which is the case normally used for the subject of a sentence. It is the one doing the action of ambulat.

So in Avia ... ambulat, avia is the person who is walking.

Latin does not have definite or indefinite articles like English the or a/an.

So:

• avia can mean the grandmother or a grandmother
• puer can mean the boy or a boy

Context tells you which is meant.

The preposition per is used to show movement through, along, or by way of something. With scalas, it gives the idea of moving along the stairs or by means of the stairs.

Also, per always takes the accusative case, which is why the noun after it is scalas.

Latin often uses scalae in the plural to mean stairs or a staircase, much like English stairs is also plural in form.

So:

• scala = a ladder, staircase, or step in some contexts
• scalae = stairs / staircase

That is why per scalas is natural Latin.

Scalas is accusative plural.

It is accusative because per takes the accusative case. So whenever you see per, the following noun should normally be accusative.

Lente is an adverb, meaning it describes how the grandmother walks.

It means slowly.

It comes from the adjective lentus, lenta, lentum. A very common way to form an adverb from a 1st/2nd declension adjective is to use -e:

• lentus = slow
• lente = slowly

Because both verbs are third person singular present active indicative.

The ending -t tells you he/she/it is doing the action:

• ambulat = he/she walks or is walking
• ducit = he/she leads or is leading

That matches the singular subjects:

• avia = one grandmother
• puer = one boy

Eam means her, and it refers back to avia.

Instead of repeating the noun, Latin often uses a pronoun:

• puer aviam ducit = the boy leads the grandmother
• puer eam ducit = the boy leads her

Using eam avoids repetition, just as English often says her instead of repeating the grandmother.

Because ducit takes a direct object, and the direct object must be in the accusative case.

For the pronoun is, ea, id:

• ea = nominative feminine singular, she
• eam = accusative feminine singular, her

Since the boy is leading her, Latin needs eam.

Yes. Latin word order is much more flexible than English word order because the endings show the grammatical roles.

So all of these are possible:

• puer eam ducit
• eam puer ducit
• ducit puer eam

They can all mean the boy leads her, though the emphasis may shift slightly.

The sentence uses puer eam ducit because it is a clear and natural order:

• subject: puer
• object: eam
• verb: ducit

Yes. In this sentence, ambulat and ducit are both in the present tense, and they naturally describe actions happening together:

• the grandmother is walking slowly
• the boy is leading her

Latin often uses the present tense this way, just as English does.

Yes. Ducit basically means leads or guides.

That makes good sense here: the boy is helping the grandmother by leading her. It suggests guidance rather than simply carrying or transporting her.

So ducit is a very natural verb in this sentence.