La sixième leçon commence.

Because leçon is a feminine singular noun in French.

• la = the for feminine singular nouns
• le = the for masculine singular nouns

So:

• la leçon = the lesson
• not le leçon

This is about grammatical gender, not biological sex. French nouns are simply classified as masculine or feminine, and leçon happens to be feminine.

Because ordinal numbers like first, second, third, sixth usually go before the noun in French.

So:

• la sixième leçon = the sixth lesson

This is the normal word order. A learner might expect something more like the lesson sixth, but French does not usually do that here.

Sixième is an ordinal number, meaning sixth.

Compare:

• six = six (cardinal number)
• sixième = sixth (ordinal number)

So sixième does not mean the quantity six; it shows the position in a sequence.

For many ordinal numbers in French, you add -ième to the cardinal number.

Examples:

• quatre → quatrième = fourth
• cinq → cinquième = fifth
• six → sixième = sixth
• sept → septième = seventh

There are a few spelling changes with some numbers, but six becomes sixième very straightforwardly.

Because sixième has the same form for masculine and feminine.

So you can say:

• le sixième chapitre = the sixth chapter
• la sixième leçon = the sixth lesson

Many ordinal numbers in French work this way. A famous exception is:

• premier = first (masculine)
• première = first (feminine)

But sixième stays sixième in both genders.

It is pronounced approximately see-ZYEM.

A more careful breakdown is:

• si-
• -zième

Important points:

• The x here is pronounced like a z sound.
• The ending -ième sounds roughly like yem.

So sixième is not pronounced like English six-ee-em.

The ç is called a cedilla. It makes the c sound like s before a, o, or u.

So:

• leçon is pronounced with an s sound: luh-SON (roughly)
• without the cedilla, c before o would normally sound like k

That is why French writes leçon and not lecon.

Because commence is the conjugated form, while commencer is the infinitive.

• commencer = to begin / to start
• commence = begins / starts

The subject is la sixième leçon, which is third person singular, so the present-tense verb form is:

• la sixième leçon commence = the sixth lesson begins

It is the present tense.

More specifically, it is the third-person singular present of commencer.

Here is part of the present-tense pattern:

• je commence = I begin
• tu commences = you begin
• il / elle commence = he / she begins
• nous commençons = we begin
• vous commencez = you begin
• ils / elles commencent = they begin

In your sentence, the subject is singular, so commence is the correct form.

Yes, several letters are not fully pronounced in the way an English speaker might expect.

A few important points:

• In leçon, the final n is not pronounced as a full n sound; it helps create a nasal vowel.
• In commence, the final e is silent.
• In sixième, you do not pronounce it letter by letter.

So French spelling and pronunciation do not match in a simple one-letter-one-sound way.

No.

Elision happens when words like la or le come before a vowel sound:

• la amie would become l’amie
• le homme would become l’homme

But here, sixième begins with a consonant sound, so the article stays:

• la sixième leçon

No contraction is needed.

Normally, no. That would sound unnatural in standard French.

With ordinal numbers, French normally puts the ordinal before the noun:

• la sixième leçon

So the natural sentence is:

• La sixième leçon commence.

A rough pronunciation guide is:

la see-ZYEM luh-SON koh-MAHNSS

A more French-like pronunciation is approximately:

la si-zjèm lə-son komãs

A few pronunciation notes:

• sixième has a z sound in the middle.
• leçon has a nasal vowel at the end.
• commence ends with a clear s sound, not a pronounced final e.

An English approximation can help at first, but listening to native audio is the best way to learn the real sound.