Vicinam miserae aviae miseret, quod illa sola tam grave onus portat.

Because miseret is an impersonal verb.

With miseret, Latin does not use a normal nominative subject for the person who feels pity. Instead:

• the person who feels pity goes in the accusative
• the person or thing pitied goes in the genitive

So:

• vicinam = the neighbor woman in the accusative = the one who feels pity
• miserae aviae = of the poor grandmother in the genitive = the one pitied

A very literal way to think of it is:

• The poor grandmother causes pity to the neighbor woman

But in natural English we say:

• The neighbor woman pities the poor grandmother

It is genitive singular.

That is because miseret takes the genitive for the person or thing that is the object/cause of pity.

So miserae aviae means:

• of the poor grandmother

Even though -ae can also be dative singular in first-declension forms, here the verb tells you it must be genitive, not dative.

Miseret is the 3rd person singular present active indicative of the impersonal verb miseret.

It means something like:

• it causes pity
• there is pity for
• idiomatically, someone pities someone

This is one of several Latin impersonal verbs of feeling that use unusual case patterns. A learner may compare it with verbs like:

• pudet = it shames
• taedet = it wearies/disgusts
• paenitet = it causes regret
• piget = it annoys / causes reluctance

So the construction here is completely normal Latin, even though it feels strange from an English point of view.

Yes.

Vicinam is the accusative singular of vicina, which means female neighbor or neighbor woman.

So this sentence is clearly talking about a woman. If it were a male neighbor, Latin would use vicinum from vicinus.

Most naturally, illa refers to avia.

So the sense is:

• the neighbor pities the poor grandmother,
• because that woman / she alone carries such a heavy burden.

Why is that the most natural reading?

Because the clause after quod explains why the neighbor feels pity, and it makes most sense that the pitied person, the grandmother, is the one carrying the burden.

Technically, since both vicina and avia are feminine singular, a pronoun could be ambiguous in isolation. But context strongly points to avia here.

Latin often leaves pronouns out when they are clear from the verb ending. So yes, Latin could often omit a subject pronoun.

But illa is useful here because there are two feminine singular people already in the sentence:

• vicina
• avia

Using illa helps point to one of them more clearly, and it can also add a slight sense of emphasis:

• that woman
• she, that one

So illa is doing more than a bare English she might do.

Sola means alone or by herself.

It agrees with illa:

• illa = feminine nominative singular
• sola = feminine nominative singular

So together they mean:

• she alone
• that woman by herself

It does not go with onus.
It describes the woman who is carrying the burden, not the burden itself.

Because quod can introduce a causal clause.

So here:

• quod illa sola tam grave onus portat
• = because she alone carries such a heavy burden

This is a very common use of quod in Latin prose.

The verb in the clause is portat, an indicative, which fits well with a straightforward statement of reason.

Because onus is a neuter noun.

Its accusative singular form is onus, and any adjective agreeing with it must also be neuter accusative singular.

So:

• onus = neuter accusative singular
• grave = neuter accusative singular of gravis

That is why Latin has:

• tam grave onus = such a heavy burden

A common beginner mistake is to expect gravem, but gravem would be masculine or feminine accusative singular, not neuter.

Yes, at least a little.

Latin word order is more flexible than English, so case endings show the grammar, while word order often helps with emphasis or flow.

In this sentence:

• Vicinam comes first, so the sentence opens with the person affected by pity
• miserae aviae follows, giving the person pitied
• miseret comes after both, completing the impersonal construction
• the quod clause then explains the reason

So the order is natural and expressive, even if it is not the order an English speaker would expect.

An English speaker may want to rearrange it mentally as:

• Vicinam miserae aviae miseret
• The neighbor pities the poor grandmother

and then add:

• quod illa sola tam grave onus portat
• because she alone carries such a heavy burden