Le petit-déjeuner précède le déjeuner.

Because they are related words. In modern standard French, petit-déjeuner means breakfast and déjeuner means lunch.

Historically, déjeuner was connected with breaking the fast, and petit-déjeuner literally means something like small breakfast/early meal. Over time, the meanings settled into the modern meal names used in France.

Because petit-déjeuner is a fixed compound noun. French often hyphenates expressions that function as one word.

So here, le petit-déjeuner is treated as a single noun meaning breakfast, not just the adjective petit plus the noun déjeuner used freely.

French often uses the definite article where English does not. So le petit-déjeuner and le déjeuner can mean breakfast and lunch in a general sense, not necessarily the breakfast and the lunch of one specific day.

That is very normal in French:

• J’aime le café. = I like coffee.
• Le petit-déjeuner précède le déjeuner. = Breakfast comes before lunch.

Yes, both are masculine singular here. You can tell because the article is le.

If they were feminine, you would see la instead.

Here it is a noun. You can tell because it comes after the article le: le déjeuner = lunch.

But déjeuner can also be a verb:

• Je déjeune à midi. = I have lunch at noon.

So this is a useful word to recognize in both roles.

Précède is the third-person singular present form of the verb précéder.

The subject is Le petit-déjeuner, which is singular, so the verb must also be singular:

• Le petit-déjeuner précède...
• not Le petit-déjeuner précèdent...

So the agreement is the same basic idea as Breakfast comes before lunch, not Breakfast come before lunch.

Précéder is completely correct, but it sounds a bit formal or textbook-like. In everyday speech, French speakers might more often say something like:

• Le petit-déjeuner vient avant le déjeuner.
• On prend le petit-déjeuner avant le déjeuner.

So the sentence is good French, but it has a slightly formal, definition-style feel.

The accents mainly help with pronunciation.

• é in déjeuner sounds like a clear ay sound.
• è in précède is a more open sound, a bit like eh.

They can also help distinguish different forms of a word. For example, précède and précéder are related, but they are not pronounced exactly the same way.

A rough English-style guide is:

luh puh-tee day-zhuh-nay pray-SEHD luh day-zhuh-nay

A few notes:

• j in déjeuner sounds like the s in measure
• the r in French is not like a strong English r
• the eu sound in -jeu- does not exist exactly in English

So if you want a more French-like version, think:

lə p(ə)-ti de-zhø-nay pre-SEHD lə de-zhø-nay

Because petit-déjeuner is a fixed compound noun. You are not freely describing a déjeuner as small in this sentence; you are using the established word for breakfast.

So learners should treat petit-déjeuner as one vocabulary item.

Not always in every French-speaking region. In France, déjeuner usually means lunch, which matches this sentence.

But in some other places, such as Quebec, Belgium, or parts of Switzerland, meal names can be different. For example, déjeuner may mean breakfast there. So this sentence reflects the standard usage of France French.