Omnis familia in atrio patrem redeuntem exspectat.

Because familia is a collective noun in Latin, just like family in English. Grammatically it is singular, even though it refers to a group made up of multiple people.

So:

• familia = family / household
• subject = singular
• verb = singular

That is why the sentence has exspectat (waits / is waiting) rather than a plural verb.

Because familia is singular, the adjective modifying it must also be singular.

• omnis = singular
• omnes = plural

So:

• omnis familia = the whole family or every family depending on context
• omnes familiae would be all the families

Here it means the whole family, so omnis familia is correct.

Familia is nominative singular, because it is the subject of the sentence—the one doing the action of waiting.

A quick way to see that:

• familia is the thing that exspectat
• the subject of a Latin finite verb is normally in the nominative

So familia is nominative singular feminine.

Because exspectat is a transitive verb: someone waits for someone or something. In Latin, the direct object of a transitive verb is put in the accusative.

So:

• familia = subject, nominative
• patrem = object, accusative

That is why you get patrem rather than pater.

Redeuntem is the present active participle of redeo, redire, meaning returning.

It describes patrem, so patrem redeuntem means the father returning or the father as he is coming back.

A participle is a verbal adjective: it has verbal meaning, but it behaves like an adjective and agrees with the noun it describes.

Because the participle has to agree with patrem in case, number, and gender.

Patrem is:

• masculine
• singular
• accusative

So the participle must also be:

• masculine
• singular
• accusative

That gives redeuntem.

If it were nominative masculine singular, it would be rediens. But here the word is modifying patrem, which is accusative, so Latin uses redeuntem.

Latin often uses participles where English might prefer a relative clause.

So these are similar in sense:

• patrem redeuntem exspectat = is waiting for the father returning
• patrem qui redit exspectat = is waiting for the father who is returning

The participle is more compact and very common in Latin.

Because in with the ablative usually expresses location: in / on somewhere.

So:

• in atrio = in the atrium
• this answers where?

By contrast, in with the accusative usually expresses motion into something:

• in atrium = into the atrium

Here the family is already there, waiting, so Latin uses in atrio.

Atrio is ablative singular.

The ending -o in a second-declension neuter noun like atrium can be dative or ablative singular, but here the preposition in tells you it must be ablative, because it expresses location.

So:

• dictionary form: atrium
• here: in atrio = ablative singular

Latin word order is more flexible than English word order because the endings show the grammatical roles. That lets Latin place words for emphasis or style.

Putting the verb at the end is very common in Latin prose, especially in simple teaching sentences.

So this order:

• Omnis familia in atrio patrem redeuntem exspectat

is perfectly normal Latin, even though English would usually say:

• The whole family is waiting in the atrium for the returning father.

In isolation, omnis familia could potentially suggest every family, but in this sentence the context makes the whole family the natural meaning.

Latin omnis can mean:

• every with a singular noun in some contexts
• all / whole in others

Here, since one specific family is waiting for one specific father, the whole family is the right interpretation.

Latin has no definite article like English the and no indefinite article like a/an.

Whether a noun is understood as the family, a family, the father, or a father depends on context.

So:

• familia can mean family, a family, or the family
• patrem can mean a father or the father

In this sentence, the context makes the whole family and the father the natural translation.

You can break it down like this:

• Omnis familia = subject: the whole family
• in atrio = prepositional phrase: in the atrium
• patrem redeuntem = object + modifier: the returning father
• exspectat = verb: waits for / is waiting for

So the grammatical skeleton is:

subject + place + object + verb

That is a very common and useful way to analyze Latin sentences.