Perfectum rem factam ostendit, futurum autem id quod postea fiet.

They are being used as nouns, not just as ordinary adjectives.

Latin often does this with grammatical terms. Here perfectum means the perfect tense and futurum means the future tense. You can think of them as shorthand for something like tempus perfectum and tempus futurum. Since tempus is a neuter singular noun, the adjectives appear in the neuter singular too.

So:

• perfectum = the perfect
• futurum = the future

Here it is understood as present tense: shows.

The form ostendit can look ambiguous in isolation, because the same spelling can represent either he/she/it shows or he/she/it showed/has shown. But the context makes the present meaning the natural one here, because this is a general grammatical statement:

• The perfect shows...
• the future, however, shows...

That kind of definition is normally stated in the present.

Both words are accusative singular feminine.

They go together as a phrase:

• rem = thing / matter
• factam = done / having been done

This phrase is the direct object of ostendit:

• Perfectum rem factam ostendit = The perfect shows a thing done

The adjective/participle factam agrees with rem in:

• gender: feminine
• number: singular
• case: accusative

Factam is the perfect passive participle of facio.

Its basic idea is done or having been done. Because it agrees with rem, the phrase rem factam literally means a thing having been done, that is, a completed thing/action.

This is a compact Latin way to express what English might say more analytically:

• something that has been done
• a completed action

Because Latin often expresses ideas very concretely.

Instead of using a highly abstract phrase, it can say something like a thing done. In grammatical explanations, this is a natural way to describe what the perfect tense points to: an action viewed as already completed.

So rem factam is not odd Latin. It is a straightforward descriptive phrase.

Autem means something like:

• however
• but
• on the other hand

It marks contrast between the two halves:

• perfectum ...
• futurum autem ...

Latin conjunctions like autem are often postpositive, which means they tend to come after the first word of their clause rather than at the very beginning. So futurum autem is more natural in Latin than putting autem first.

Because Latin often omits a word when it is easily understood from the previous clause. This is called ellipsis.

So the full sense is:

• Perfectum rem factam ostendit
• futurum autem id quod postea fiet ostendit

But Latin does not need to repeat ostendit, because the reader can supply it automatically.

This phrase means that which will happen later or the thing that will happen afterwards.

Its parts are:

• id = that, that thing
• quod = a relative pronoun, which / that
• postea = afterwards, later
• fiet = will happen / will be done

So id is the main word, and quod postea fiet explains it:

• id = that
• quod postea fiet = which will happen later

Together: that which will happen later

Because quod here is a relative pronoun, not an interrogative word.

• quid? = what?
• quod = which / that which

In this sentence, it is not asking a question. It is connecting back to id:

• id quod = that which

Since id is neuter singular, the relative pronoun is also neuter singular: quod.

Fiet is the future tense of fio.

In this sentence it means will happen or will come to be. That fits the idea of the future tense very well: it points to something that has not happened yet, but will happen later.

So:

• postea fiet = will happen later

This is a good choice because the sentence is contrasting:

• the perfect: something already done
• the future: something still to happen

Latin word order is much freer than English word order because grammatical endings show the relationships between words.

This sentence is arranged for clarity and contrast:

• Perfectum ...
• futurum autem ...

That puts the two tense names in a parallel position and makes the contrast very easy to see.

English usually depends heavily on word order for grammar. Latin can use word order more for emphasis, balance, and style.