Learn Hardcore Latin: Frater dicit se multos convivas invitaturum esse, si domus satis magna erit. - The brother says that he will invite many guests if the house is large enough.

Latin has no articles. So frater can mean brother, a brother, or the brother, depending on context. The same is true of domus and convivas. English has to choose an article, but Latin usually leaves that to context.

What is the basic structure of the sentence?

It has three main parts:

Frater dicit = the main clause, Brother says
se multos convivas invitaturum esse = the indirect statement, that he will invite many guests
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

Why doesn’t Latin use a word for that after dicit?

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