Avus dicit talem usum etiam scriptori utilem esse, quia omnia quae videt postea describere potest.

Because after dicit Latin often uses an indirect statement, and the normal pattern is:

verb of saying/thinking + accusative subject + infinitive

So here:

• Avus dicit = Grandfather says
• talem usum ... utilem esse = that such a practice/use is useful

In that construction:

• the subject of the indirect statement goes into the accusative: talem usum
• the verb becomes an infinitive: esse
• any predicate adjective agreeing with that subject also goes into the accusative: utilem

So Latin does not say talem usus utilis est here. It says talem usum utilem esse because it is dependent on dicit.

Talem usum means such a use or such a practice/habit.

It has two words that agree:

• talem = such, accusative singular masculine
• usum = use, practice, habit, accusative singular masculine

They are accusative because talem usum is the subject of the indirect statement after dicit.

A learner may expect a nominative subject, but in indirect statement Latin uses the accusative instead.

It is a noun here: usus, usūs = use, practice, experience.

In this sentence, usum is the accusative singular form of that noun.

That can confuse learners because usus can also look like a participle form in other contexts. But here the combination talem usum clearly shows a noun phrase: such a use/practice.

Because utilis often takes the dative of the person for whom something is useful.

So:

• utilis alicui = useful to someone

Here:

• scriptori = to a writer
• utilem agrees with usum, not with scriptori

So the structure is:

talem usum etiam scriptori utilem esse
= that such a practice is useful also to a writer

Because utilem agrees with usum, not with scriptori.

• usum is accusative singular masculine
• therefore the agreeing adjective is also accusative singular masculine: utilem

Meanwhile scriptori is dative because it depends on utilis in the sense useful to a writer.

So the grammar is:

• talem usum = the thing that is useful
• scriptori = the person to whom it is useful
• utilem = adjective describing usum

Etiam usually means also, even, or too.

Here it most naturally emphasizes scriptori:

etiam scriptori = also to a writer / even to a writer

So the idea is that this kind of practice is useful not only in general, but also for a writer.

Like many Latin adverbs, etiam is flexible in position, so you should pay attention to what it seems to be highlighting.

This means everything that he sees or more literally all the things which he sees.

Breakdown:

• omnia = all things / everything
  
  • neuter plural accusative
• quae = relative pronoun, which
  • agrees with omnia in gender and number
• videt = he sees

So:

omnia quae videt
= all the things that he sees

A useful point: quae here is neuter plural, matching omnia. Its case is determined by its role inside the relative clause. In meaning, it is the object of videt.

Because it refers back to omnia, which is plural.

• quod would be singular neuter
• quae can be neuter plural

Since omnia means all things / everything, the relative pronoun must match that antecedent in number and gender:

• omnia = neuter plural
• quae = neuter plural

So Latin says:

omnia quae videt
not omnia quod videt

The subject is not stated explicitly, so it is understood from the context.

• videt = he/she sees
• potest = he/she is able / can

In the sentence, the natural understood subject is the person being discussed, especially the writer or a general person practicing observation.

So the idea is:

because he can later describe everything he sees

Latin often leaves subject pronouns unspoken when the verb ending already shows the person.

Postea means afterward, later, or later on.

Here it goes with describere potest:

• postea describere potest = can describe later

Latin word order is flexible, so postea can stand between other words without causing trouble. You do not need to force it into the exact English position.

Because Latin relies much more on endings than on word order to show grammatical relationships.

For example:

• Avus is nominative, so it is the subject
• usum is accusative, so it belongs to the indirect statement
• scriptori is dative, so it means to a writer
• esse shows the infinitive construction after dicit

That means Latin can arrange words for emphasis or style more freely than English can.

A very literal order might feel strange in English:

Grandfather says such a practice also to-a-writer useful to be, because everything which he sees later to-describe he-is-able.

But the endings make the relationships clear. English has to rearrange the sentence much more strictly.