Frater dicit se multos convivas invitaturum esse, si domus satis magna erit.

It has three main parts:

1. Frater dicit = the main clause, Brother says
2. se multos convivas invitaturum esse = the indirect statement, that he will invite many guests
3. si domus satis magna erit = the condition, if the house is big enough

So the sentence is built as:

main verb of saying + indirect statement + conditional clause

After verbs of saying, thinking, knowing, and perceiving, Latin very often uses an accusative + infinitive construction instead of a that-clause.

So instead of saying:

Brother says that he will invite many guests

Latin says, more literally:

Brother says himself to be going to invite many guests

That is why you get:

• se = the subject of the reported statement, in the accusative
• invitaturum esse = the infinitive phrase

This is one of the most important Latin patterns to learn.

Se is the reflexive pronoun, and here it refers back to frater, the subject of dicit.

So:

• Frater dicit se... = Brother says that he...

In other words, the brother is talking about himself.

If Latin used eum instead, it would normally mean that he in the sense of some other male person, not the brother himself.

Invitaturum esse is the future active infinitive of invitare.

It is formed from:

• the future active participle: invitaturus, -a, -um
• plus esse

So:

• invitaturum esse = to be about to invite, or more naturally, to invite / will invite

In this sentence it shows action that is future relative to dicit:

• Frater dicit se... invitaturum esse = Brother says that he will invite...

Because invitaturum agrees with se, and se is accusative.

In an indirect statement, the subject is put in the accusative, so anything agreeing with that subject must also be in the accusative.

Here:

• se = accusative singular
• invitaturum = accusative masculine singular

If the subject were feminine, you would get invitaturam esse. If it were plural masculine, invitaturos esse, and so on.

Because they are the direct object of invitare.

The brother is inviting the guests, so convivas is the object.
The adjective multos agrees with convivas in case, number, and gender.

So:

• multos = many
• convivas = guests
• together: many guests

A useful extra point: conviva is a first-declension noun, but it can refer to a male or female guest. Here multos shows masculine or mixed-gender plural.

Because the condition refers to the future.

• erit = will be
• est = is

Latin often uses the future indicative in a future condition:

• si domus satis magna erit = literally if the house will be sufficiently large

In smoother English, we usually say:

• if the house is big enough

So Latin is more explicit here about future time than English usually is.

The si clause belongs with the reported idea: the brother says that he will invite many guests if the house is big enough.

A helpful way to see it is to imagine the direct version first:

Si domus satis magna erit, multos convivas invitabo.

Then Latin reports that statement indirectly:

• invitabo becomes invitaturum esse
• the subject ego becomes se

But the si clause itself stays a normal finite clause here, with erit.

Because domus is an irregular noun. It is usually treated as a fourth-declension noun, and it is feminine.

So even though -us often makes learners think of a masculine second-declension noun, domus is different.

Here domus is nominative singular because it is the subject of erit:

• domus erit = the house will be

Satis is an indeclinable adverb meaning enough or sufficiently.

It modifies magna:

• satis magna = big enough / sufficiently large

Magna is nominative feminine singular because it agrees with domus, and it is a predicate adjective after erit.

So:

• domus satis magna erit = the house will be big enough

It is not magnam because domus is not a direct object here; it is the subject.

Latin word order is flexible, because the endings show the grammatical relationships.

This order is natural and clear:

• Frater dicit
• se
• multos convivas
• invitaturum esse
• si domus satis magna erit

But Latin could rearrange parts for emphasis, for example:

Frater dicit se, si domus satis magna erit, multos convivas invitaturum esse.

That would still mean the same thing. The cases and verb forms do most of the grammatical work.

A direct version would be:

Si domus satis magna erit, multos convivas invitabo.

That means:

If the house is big enough, I will invite many guests.

Then, when Latin changes that into indirect statement after dicit:

• invitabo becomes invitaturum esse
• the original speaker’s I becomes se

So:

• direct: invitabo
• indirect: se ... invitaturum esse

This is a very useful way to understand how indirect statement works.