Alter librum legit, alter in commentario scribit.

Repeated alter ... alter ... means the one ... the other ... or one ... the other ....

So the sentence is structured as a contrast between two people:

• alter librum legit = one/the one reads a book
• alter in commentario scribit = the other writes in the notebook/commentary

Latin often uses repetition like this to set up a neat contrast.

Yes. Alter normally means the other of two.

That is an important nuance. A native English speaker may first think alter just means another, but in Latin it usually points to a pair:

• alter = the other one, when there are two
• alius = another, a different one, often without the strong one of two idea

So alter ... alter ... strongly suggests two people are involved: one does this, the other does that.

Because librum is the accusative singular, used for the direct object.

Here, the book is the thing being read, so Latin puts liber into the accusative:

• liber = book, as a subject
• librum = book, as a direct object

Compare:

• Liber cadit = the book falls
• Puer librum legit = the boy reads the book

In your sentence, librum is the object of legit.

Commentario is ablative singular.

It is ablative because it follows in in the sense of location:

• in + ablative = in, on, inside, within

So:

• in commentario = in the notebook / in the commentary

This tells you where the writing is happening.

Because Latin is saying writes in the notebook/commentary, not writes the notebook/commentary.

That distinction matters:

• in commentario scribit = he/she writes in the notebook
• commentarium scribit = he/she writes a commentary or writes a notebook/document

So the preposition in shows location or place, not a direct object.

It can mean several related things, depending on context, such as:

• notebook
• memorandum book
• commentary
• notes

In a simple learning sentence like this, it is often best understood as notebook or exercise book, because that fits naturally with scribit.

So in commentario scribit most likely means writes in a notebook.

Yes, in this sentence it is understood as present tense: reads.

A slightly tricky point is that legit can look the same as a perfect form in some contexts, but here the parallel structure makes the present meaning clear:

• legit = reads
• scribit = writes

Since both clauses describe what the two people are doing, the natural reading is present tense in both.

Latin often leaves subject pronouns out because the verb ending already shows the person and number.

For example:

• legit = he/she/it reads
• scribit = he/she/it writes

Also, in this sentence the subjects are already expressed by alter ... alter ..., so extra pronouns would be unnecessary.

English usually needs a subject pronoun, but Latin often does not.

It is normal and natural Latin word order.

Latin word order is more flexible than English word order because case endings show each word’s job in the sentence. Still, there are common patterns, and one very common one is to place the verb near the end of the clause.

So:

• Alter librum legit
• alter in commentario scribit

feels quite ordinary in Latin.

English depends heavily on word order, but Latin depends much more on endings like -um and -o to show function.

It could be translated that way in smooth English, but one ... the other ... is more exact.

That is because alter usually refers to the other of two, not just any other person. So the closest translation is:

• one reads a book, the other writes in a notebook

If you say another, the English is understandable, but it weakens the idea that there are specifically two people.

Latin does not always need et when two short parallel clauses are simply placed next to each other.

The comma already helps show the relationship, and the repeated alter makes the contrast very clear.

So Latin can say:

• Alter librum legit, alter in commentario scribit

without needing et.

In English, we also sometimes do something similar: One reads a book, the other writes in a notebook.