Familia cometen et sidus erraticum contemplatur.

Because familia is a singular collective noun in Latin. It refers to a group, but grammatically it is treated as one unit, so it takes a singular verb: contemplatur.

This is similar to English the family is in standard American English, though English can vary more than Latin does.

Familia is in the nominative singular, which is the normal case for the subject.

You can also tell from the rest of the sentence:

• contemplatur is 3rd person singular, matching familia
• cometen is an object form
• sidus erraticum is also functioning as an object

So familia is the one doing the action.

Because contemplor, contemplari is a deponent verb.

Deponent verbs use passive-looking forms but have active meanings. So:

• contemplatur does not mean is being watched
• it means watches, observes, or contemplates

This is a very common thing in Latin, and it often surprises English speakers at first.

Cometen is a Greek-style accusative singular form. The word for comet is a Greek loanword in Latin, and Latin sometimes keeps Greek endings for such nouns.

So here cometen is the direct object of contemplatur.

In other texts, you may also see a more fully Latinized form such as cometam. The sentence you have uses the Greek-style form.

Because sidus is a neuter third-declension noun, and in Latin neuter nominative and accusative singular are identical.

So:

• nominative singular: sidus
• accusative singular: sidus

That is normal for neuter nouns. The word has not failed to decline; this just happens to be one of those forms where the two cases look the same.

Because erraticum is an adjective agreeing with sidus.

Since sidus is:

• neuter
• singular
• here accusative (as the object)

the adjective must match it, so we get erraticum.

Latin adjectives agree with the nouns they describe in gender, number, and case.

It is connecting two direct objects:

• cometen
• sidus erraticum

The sentence has:

• one subject: familia
• one verb: contemplatur
• two things being observed, joined by et

So the family is observing both things.

Because Latin word order is much more flexible than English word order.

Both sidus erraticum and erraticum sidus are possible in Latin. The form of the adjective shows that it belongs with sidus, so Latin does not need a fixed adjective position the way English usually does.

Noun + adjective is a very normal Latin order.

Putting the verb at the end is a very common Latin pattern, especially in straightforward prose.

So Familia cometen et sidus erraticum contemplatur has a very natural Latin feel. But Latin word order is flexible, and the words could be rearranged for emphasis without changing the basic meaning.

The endings, not just the position, tell you how the sentence works.

Because Classical Latin has no articles.

So familia can mean:

• the family
• a family

and context tells you which is best in translation. The same is true for cometen and sidus erraticum. Latin usually leaves that distinction unstated unless something in the sentence makes it clear.