Mater dicit se, antequam ad macellum eat, semper nummos numerare.

Because Latin is using an indirect statement after dicit. In this construction, the subject of the reported action goes into the accusative, and the verb goes into the infinitive.

So in se ... numerare, se means herself and refers back to mater. Literally, the structure is something like:

Mother says herself to count money...

In natural English, that becomes:

Mother says that she counts money...

After verbs of saying, thinking, knowing, hearing, and similar verbs, Latin very often uses the accusative-and-infinitive construction instead of a clause with that.

So:

• Mater dicit se numerare = Mother says that she counts
• not usually Mater dicit quod numerat in classical style

Here:

• dicit = the main verb, says
• se = subject of the reported action
• numerare = infinitive, to count

Together they express that she counts.

Because se is a reflexive pronoun. In Latin, reflexive pronouns normally refer back to the subject of their own clause.

Here the main subject is mater, so se refers back to mater.

If Latin wanted to say her meaning some other female person, it would normally use eam, not se.

So:

• mater dicit se numerare = mother says that she herself counts
• mater dicit eam numerare = mother says that she/that woman counts

Eat is the present subjunctive of eo, ire (to go).
It is the present indicative.

After antequam (before), Latin often uses the subjunctive when the action is viewed as still ahead or anticipated from the point of view of the sentence.

So:

• antequam ... eat = before she goes

This is very normal Latin.

They are two different moods of the same verb:

• it = indicative, she goes
• eat = subjunctive, roughly she go / may go / should go, depending on context

In this sentence, the subjunctive is not best translated with a separate English word. English just says:

before she goes to the market

But Latin shows the nuance through the subjunctive.

It tells us when she counts the money. In other words, it modifies the action numerare.

So the sense is:

Mother says that she always counts the money before she goes to the market.

The before clause belongs to the content of what she says, not to the act of saying itself.

Because ad means to, toward, and it takes the accusative case.

So:

• ad macellum = to the market

Here macellum is accusative singular.

Macellum is a market, especially a market for food or provisions. It is not the most general word for every kind of marketplace, but in many beginner texts it is simply translated as market.

So ad macellum is naturally to the market.

Nummos is accusative plural, the direct object of numerare.

• nummus = coin
• nummos = coins

In context, nummos numerare can mean literally to count coins, or more loosely to count money.

There is no separate word for that here, because Latin usually does not need one in indirect statement.

English says:

Mother says that she counts...

Latin says:

Mater dicit se ... numerare

So the combination accusative + infinitive does the work that that does in English.

Yes, it could. Latin word order is much freer than English word order.

Semper is an adverb meaning always, and it modifies numerare. Its position here is perfectly natural, but Latin could move it for emphasis without changing the basic meaning very much.

So the important point is not its exact position, but what it modifies:

she always counts the money

No. Latin does not have a word for the or a/an.

So mater can mean:

• mother
• the mother
• sometimes a mother

You decide from context which English wording is best. In this sentence, Mother or the mother both make sense depending on context.

The infinitive itself does not tell you that. The word that gives the habitual sense is semper.

Because of semper, the sentence means this is her regular practice:

she always counts the money before she goes to the market

So the habitual meaning comes mainly from semper, not from the infinitive form alone.