Avia per scalas tardius descendit quam puer.

Avia is nominative singular. It means grandmother and is the subject of the verb.

In a basic Latin sentence, the noun in the nominative case is usually the one doing the action. Here, avia is the one who descendit.

Per takes the accusative case, so scalas is accusative plural.

So:

• per = through, along, by way of
• scalas = stairs in the accusative plural

Together, per scalas means something like down the stairs, by way of the stairs, or more literally through/along the stairs.

Latin often uses the plural scalae for stairs, just as English often says stairs rather than a stair.

So:

• scala = a ladder or a single stair/step in some contexts
• scalae = stairs, stairway

That is why per scalas is a very natural way to say down the stairs.

Because tardius is an adverb, while tardior is an adjective.

Here the sentence is describing how the grandmother goes down the stairs, so Latin needs an adverb:

• tardus = slow
• tarde = slowly
• tardius = more slowly

By contrast:

• tardior = slower

So:

• avia tardior est quam puer = the grandmother is slower than the boy
• avia tardius descendit quam puer = the grandmother goes down more slowly than the boy

It is the comparative adverb of tarde.

A common pattern is:

• adjective: tardus = slow
• adverb: tarde = slowly
• comparative adverb: tardius = more slowly

Many comparative adverbs end in -ius.

It means than the boy.

More fully, the idea is:

Avia per scalas tardius descendit quam puer descendit.

Latin often leaves out words that are obvious from context, especially repeated verbs. So quam puer really means:

than the boy does

Because puer is understood as the subject of an omitted verb:

quam puer descendit

Since puer is the subject of that implied verb, it stays in the nominative.

A useful way to think about it is that quam often introduces a comparison where Latin can leave the second half shortened, but the case still matches what the full clause would require.

It can mean either one, depending on context.

For this verb, the form descendit can be:

• present tense: he/she descends
• perfect tense: he/she descended

So by itself, the form is ambiguous. In an isolated sentence, you need context to know which is meant. If the meaning has already been given to the learner, then that settles it for this sentence.

Latin often puts the verb at the end, but it does not have to. Word order in Latin is much more flexible than in English.

This sentence places the comparison quam puer after the verb:

Avia per scalas tardius descendit quam puer.

That is perfectly normal Latin. The sentence still clearly means that the grandmother goes down the stairs more slowly than the boy.

Very literally, per often means through or along, but in natural English that would sound strange here.

So although the literal pieces are:

• per = through / along / by way of
• scalas = stairs

the natural sense is down the stairs or by the stairs/stairway. In this sentence, down the stairs is the best idiomatic translation.

Very often, yes. In a sentence like this, quam introduces the second part of the comparison:

• tardius ... quam puer = more slowly ... than the boy

Latin also has another way to make comparisons, sometimes using the ablative without quam, but with this sentence the comparison is made with quam.

A helpful breakdown is:

• Avia = grandmother
• per scalas = down the stairs
• tardius = more slowly
• descendit = descends / descended
• quam puer = than the boy / than the boy does

So the grammar is basically:

subject + prepositional phrase + comparative adverb + verb + comparison

That makes it a very useful model sentence for learning how Latin expresses comparisons of manner.