Je nettoie le grille-pain après le petit-déjeuner.

Because nettoie is the conjugated form of the verb nettoyer in the present tense.

The infinitive is nettoyer = to clean.

In the present tense:

• je nettoie = I clean / I am cleaning
• tu nettoies
• il/elle nettoie
• nous nettoyons
• vous nettoyez
• ils/elles nettoient

So je nettoyer would be like saying I to clean, which does not work.

Also, this verb has a spelling change:

• before a silent ending, y often becomes i
• so nettoyer → je nettoie

In French, nouns usually need an article, such as le, la, les, un, or une.

So French says:

• je nettoie le grille-pain

where English often says:

• I clean the toaster

Here, le means the.

French uses articles more often than English, even in cases where English might sometimes leave them out.

You know it is masculine because the sentence uses le grille-pain.

French nouns have grammatical gender, and grille-pain happens to be masculine:

• un grille-pain
• le grille-pain

There is not always a logical reason that helps learners; often you simply have to learn the noun together with its article.

A good habit is to memorize:

• le grille-pain = the toaster

not just grille-pain by itself.

Grille-pain is a compound noun, made from two parts:

• grille from griller = to grill / toast
• pain = bread

Together, grille-pain literally suggests something like bread-griller, which is why it means toaster.

Many French compound nouns use a hyphen, so this is normal spelling.

For the same general reason: petit-déjeuner is also a compound word.

It literally comes from:

• petit = small
• déjeuner

As a noun, le petit-déjeuner means breakfast.

French commonly writes this meal name with a hyphen when it is a noun:

• le petit-déjeuner = breakfast

Because in this sentence petit-déjeuner is a noun, and French normally uses an article with nouns.

So:

• après le petit-déjeuner = after breakfast

Even though English often says simply after breakfast, French usually keeps the article:

• avant le dîner
• après le déjeuner
• pendant le repas

This is a very common difference between English and French.

Après means after. It is a preposition.

It introduces the time expression:

• après le petit-déjeuner = after breakfast

So the sentence structure is:

• Je nettoie = I clean / I am cleaning
• le grille-pain = the toaster
• après le petit-déjeuner = after breakfast

It can mean either one, depending on context.

The French present tense often covers both:

• I clean the toaster after breakfast
• I am cleaning the toaster after breakfast

Without more context, French does not force the same distinction English often makes between simple present and present continuous.

Very often, this sentence would be understood as a habitual action:

• I clean the toaster after breakfast

But in the right context, it could also describe what is happening now.

French uses j’ only before a word beginning with a vowel sound or a silent h.

Examples:

• j’aime
• j’habite

But nettoie begins with the consonant sound n, so there is no elision:

• je nettoie

Not:

• j’nettoie

A helpful rough guide is:

• je sounds like zhuh
• nettoie sounds roughly like neh-twah

So je nettoie is approximately:

• zhuh neh-twah

A few notes:

• the j in je is the soft French sound heard in measure
• the ending -toie sounds like twah
• French pronunciation is smoother and less strongly stressed than English

A rough English-friendly guide is:

• gree-pahn

A few details:

• grille sounds close to gree
• pain does not sound like English pain
• French pain has a nasal vowel, something like pahn said through the nose

So:

• le grille-pain ≈ luh gree-pahn

A rough guide is:

• puh-tee day-zhuh-nay

But in real French speech, it may sound more compressed and flowing than that.

Useful points:

• petit = roughly puh-tee
• déjeuner has the soft j sound, like the s in measure
• the final -er sounds like ay

So:

• le petit-déjeuner ≈ luh puh-tee day-zhuh-nay

Because French generally requires articles more often than English.

English might say:

• I clean the toaster after breakfast

French says:

• Je nettoie le grille-pain après le petit-déjeuner.

This is completely normal French. Learners often want to drop le, but in standard French that usually sounds incomplete or wrong.

So a good rule is:

• when in doubt, French probably wants an article before the noun

Yes. That would mean:

• after having breakfast
• more naturally in English, after eating breakfast

So you could say:

• Je nettoie le grille-pain après le petit-déjeuner.
• Je nettoie le grille-pain après avoir pris le petit-déjeuner.

The first is simpler and more direct.
The second is a bit more explicit about the action of having eaten breakfast.

By itself, it can be either, but it most naturally suggests a habit or routine.

That is because:

• the present tense in French often describes regular actions
• après le petit-déjeuner sounds like part of a routine

So many listeners would understand:

• I clean the toaster after breakfast
  as something the speaker usually does.

If you wanted to make the one-time meaning clearer, context would help, or you could use a different tense.

Yes, related forms exist, but in this sentence petit-déjeuner is clearly a noun meaning breakfast.

As a verb, petit-déjeuner can mean to have breakfast, though in everyday modern French, people very often say:

• prendre le petit-déjeuner or simply
• prendre son petit-déjeuner

So here:

• le petit-déjeuner = the meal, breakfast

not the action as a verb.