Decima lectio incipit.

Because it agrees with lectio, which is a feminine singular noun.

Decima is the feminine nominative singular form of the ordinal adjective meaning tenth. Latin adjectives usually match the noun they describe in gender, number, and case.

So:

• decimus = masculine
• decima = feminine
• decimum = neuter

Since lectio is feminine, Latin uses decima lectio.

It is nominative singular.

That is because lectio is the subject of the sentence, and the subject of a finite verb is normally in the nominative case. Since decima describes lectio, it also appears in the nominative singular feminine.

So the structure is:

• decima lectio = the subject
• incipit = the verb

Lectio is a third-declension feminine noun.

Its basic dictionary form is:

• lectio, lectionis = lesson, reading

A learner may notice that the stem changes:

• nominative singular: lectio
• other forms often use lection-

For example:

• lectionis = of the lesson
• lectionem = lesson (object)

That is normal for many third-declension nouns.

Because Classical Latin does not have definite or indefinite articles like English the or a/an.

So decima lectio can mean:

• the tenth lesson
• a tenth lesson

In most contexts, the meaning is clear from the situation. In a textbook heading, the tenth lesson is the natural translation.

Incipit is:

• third person singular
• present tense
• active voice
• indicative mood

It comes from incipio, incipere, meaning to begin or to start.

So incipit means:

• begins
• is beginning

Because the subject lectio is singular, the verb is singular too.

Because the subject is singular: decima lectio = the tenth lesson.

In English:

• the lesson begins
• the lessons begin

Latin shows this distinction in the verb ending:

• incipit = he/she/it begins
• incipiunt = they begin

Since lectio is one lesson, Latin uses incipit.

Not very. Latin word order is more flexible than English word order because the endings show how words function.

So these could all mean essentially the same thing:

• Decima lectio incipit
• Lectio decima incipit
• Incipit decima lectio

That said, decima lectio incipit is a perfectly natural order, especially in a simple textbook sentence.

Yes. Lectio can also mean reading.

Its basic idea is connected with reading or a reading passage, and in educational contexts it often comes to mean lesson. So depending on context, decima lectio might be understood as:

• the tenth lesson
• the tenth reading

In a Latin textbook, the tenth lesson is usually the best choice.

A rough Classical pronunciation would be:

DEK-i-ma LEK-ti-o IN-ki-pit

A few helpful points:

• c is always hard, like k
• ti in lectio is usually pronounced clearly as ti-o
• incipit has a hard c as well

So:

• Decima ≈ DEK-ee-ma
• lectio ≈ LEK-tee-o
• incipit ≈ IN-ki-pit

Because the verb ending already includes that information.

In incipit, the -t ending tells you the verb is third person singular: he/she/it begins. Since lectio is the subject, English uses it begins, but Latin does not need a separate pronoun.

This is very common in Latin:

• amat = he/she/it loves
• videt = he/she/it sees
• incipit = he/she/it begins