Antequam ad mensam sedeamus, mater dicit manus lavandas esse.

Because antequam can introduce a clause about something that has not happened yet and is still only anticipated. In that kind of before clause, Latin often uses the subjunctive.

So:

• antequam ad mensam sedeamus = before we sit down at the table
• literally, it has the feel of before we are about to sit down

If the action were being presented more as a simple factual event, Latin could use the indicative in some contexts, but here the subjunctive fits the idea of an action still in prospect.

Sedeamus is:

• present subjunctive
• 1st person plural
• from sedere = to sit

So it means we may sit, let us sit, or in this sentence simply we sit / sit down in a subordinate clause after antequam.

In context:

• ad mensam sedeamus = we sit down at the table

Ad + accusative often shows movement toward something. With verbs like sit down, Latin commonly uses ad mensam.

So:

• ad mensam = to the table / at the table in the sense of taking one’s place there
• sedere ad mensam = to sit down at the table

This is more natural in Latin than trying to translate English word-for-word.

Because Latin normally uses indirect statement after verbs of saying, thinking, knowing, and perceiving.

With dicit = she says, Latin typically follows with:

• an accusative subject
• plus an infinitive

So:

• mater dicit manus lavandas esse
• literally: mother says the hands to-be-washed to be
• natural English: mother says that the hands must be washed

Latin usually prefers this accusative-and-infinitive construction instead of a that-clause.

Because esse is the infinitive needed for the indirect statement.

The phrase lavandas esse is a passive periphrastic infinitive, meaning:

• to be needing to be washed
• more naturally: must be washed

Since dicit introduces indirect statement, the verb inside that statement has to be an infinitive, so esse is required.

It is a passive periphrastic, which expresses necessity or obligation.

It is made from:

• a gerundive: lavandas
• plus esse

So:

• lavandas esse = to have to be washed / must be washed

This is one of the standard Latin ways to express must.

Because lavandas agrees with manus.

Here manus means hands, and it is:

• feminine
• plural
• accusative in indirect statement

So the gerundive must match it:

• feminine
• plural
• accusative

That gives lavandas.

So:

• manus lavandas esse = that the hands must be washed

Because in Latin indirect statement, the subject of the infinitive goes into the accusative.

Main verb:

• mater dicit = mother says

Indirect statement:

• manus lavandas esse = that the hands must be washed

Inside that indirect statement, manus is the logical subject of lavandas esse, but grammatically it appears in the accusative because that is how indirect statement works in Latin.

Good question. Manus is a fourth-declension noun, and some of its forms are identical.

For manus, manus:

• nominative singular: manus
• accusative singular: manum
• nominative plural: manus
• accusative plural: manus

So in this sentence, the form manus could look nominative or accusative, but the grammar tells you it is accusative plural, because it is the subject of an infinitive in indirect statement.

Yes, but Latin leaves it unstated.

The passive periphrastic can name the person responsible with a dative of agent, but here there is no dative. So the sentence simply expresses a general obligation:

• the hands must be washed

From the context, we understand that we are the ones who need to wash them, but Latin does not say this explicitly.

If Latin wanted to be more explicit, it could add something like a dative, but here the responsibility is left general.

Because Latin often leaves out possessives when they are obvious from context, especially with body parts.

So:

• manus lavandas esse naturally means that the hands must be washed
• in context, this really means our hands

Latin does not need to spell out our if the situation already makes that clear.

The present tense dicit can do either, depending on context.

It can mean:

• mother says = right now, in this situation
• or mother says = mother is in the habit of saying

In this sentence, most readers will probably take it as a normal present in a scene:

• Before we sit down at the table, mother says that the hands must be washed.

It has two main parts:

1. Antequam ad mensam sedeamus

   • before we sit down at the table

2. mater dicit manus lavandas esse

   • mother says that the hands must be washed

So the overall structure is:

• temporal subordinate clause
• followed by a main clause
• and inside the main clause, an indirect statement

That makes it a very useful sentence for seeing several common Latin constructions at once.