Li legas religian libron.

Li means he. It is the third-person singular pronoun for a male person.

Examples:

• Li legas. = He reads / He is reading.
• Ŝi legas. = She reads / She is reading.

In Esperanto, -as marks the present tense.

So:

• legi = to read
• legas = reads / is reading

Esperanto verb endings are very regular:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = command / wish
• -i = infinitive

So Li legas means He reads or He is reading, depending on context.

The -n ending marks the direct object in Esperanto. The direct object is the thing directly affected by the action.

In Li legas religian libron, the thing being read is libron (book), so it takes -n.

Compare:

• Li legas libron. = He is reading a book.
• La libro estas interesa. = The book is interesting.

In the second sentence, libro is not the object, so there is no -n.

Because adjectives in Esperanto must agree with the nouns they describe.

The noun is:

• libro = book
• libron = book as a direct object

The adjective is:

• religia = religious
• religian = religious agreeing with a direct object noun

So:

• religia libro = a religious book
• religian libron = a religious book as the object of the sentence

This agreement happens in number and case:

• bona libro = a good book
• bonan libron = a good book as object
• bonaj libroj = good books
• bonajn librojn = good books as object

Esperanto has la for the, but it has no article for a/an.

So:

• libro can mean a book or just book in a general sense
• la libro means the book

That means:

• Li legas religian libron. = He is reading a religious book
• Li legas la religian libron. = He is reading the religious book

If there is no la, the meaning is usually indefinite unless context suggests otherwise.

It can mean either one. Esperanto present tense does not force the same distinction that English often does.

So Li legas religian libron could mean:

• He reads a religious book
• He is reading a religious book

Context tells you which is meant.

In Esperanto, adjectives usually come before the noun, just like in English:

• religia libro = religious book

But Esperanto word order is fairly flexible, so libron religian is also possible, though less neutral in ordinary usage.

The most normal version here is:

• religian libron

The dictionary forms are:

• li = he
• legi = to read
• religia = religious
• libro = book

The sentence uses changed forms:

• legi → legas for present tense
• religia → religian because it describes an object noun
• libro → libron because it is the direct object

Yes. Because Esperanto marks the direct object with -n, word order is more flexible than in English.

For example, these can mean the same thing:

• Li legas religian libron.
• Religian libron li legas.

The second version gives extra emphasis to religian libron. The first version is the most neutral and standard.

You can tell because the adjective and noun match:

• religian = adjective
• libron = noun
• both have the -n ending

That matching shows that religian describes libron. Together they form the object phrase:

• religian libron = a religious book

This agreement is one of the features that makes Esperanto sentence structure very clear.