Je ne suis pas disponible avant midi, mais tu peux appeler Marie.

This is the standard way to make a sentence negative in French.

• Je suis disponible = I am available
• Je ne suis pas disponible = I am not available

In simple tenses, ne goes before the conjugated verb and pas goes after it:

• je ne suis pas
• tu n’es pas
• il ne parle pas

In everyday spoken French, people often drop ne and say:

• Je suis pas disponible

But in standard written French, ne ... pas is the normal form.

Because être is the infinitive, meaning to be. In a sentence, you usually need a conjugated form, not the infinitive.

Here, the subject is je, so être becomes suis:

• je suis = I am
• tu es = you are
• il/elle est = he/she is

So:

• Je suis disponible = I am available

not

• Je être disponible ✘

Because disponible agrees with the subject, and here the subject is singular: je.

• singular: disponible
• plural: disponibles

Examples:

• Je suis disponible. = I am available.
• Nous sommes disponibles. = We are available.

Also, disponible has the same form in the masculine and feminine singular, so whether the speaker is male or female, disponible stays the same here.

Avant midi literally means before noon.

• avant = before
• midi = noon / midday

French often uses time expressions like this without an article:

• avant midi = before noon
• après midi appears in other contexts, though après-midi as a noun means afternoon

Here, midi is being used as a fixed time point, so no article is needed.

French uses midi for noon or midday. It is the normal everyday word.

Examples:

• à midi = at noon
• avant midi = before noon
• vers midi = around noon

So even if English might say before noon or before midday, French naturally uses midi.

Mais means but. It connects two contrasting ideas:

• Je ne suis pas disponible avant midi = I’m not available before noon
• mais tu peux appeler Marie = but you can call Marie

So mais introduces an alternative or contrast.

Because tu is the informal singular word for you.

French has different ways to say you:

• tu = informal singular
• vous = formal singular or plural

So:

• tu peux appeler Marie = you can call Marie
  (said to one person informally)

If you wanted a formal version, you would say:

• vous pouvez appeler Marie

Because pouvoir (to be able to / can) is a modal verb. In French, when you use a modal verb, you conjugate the first verb and leave the second one in the infinitive.

Here:

• tu peux = you can
• appeler = call

So:

• tu peux appeler Marie = you can call Marie

This works like English:

• you can call
• not you can calls

Other examples:

• Je veux partir. = I want to leave.
• Nous devons attendre. = We must wait.

Because appeler usually takes a direct object in French.

• appeler quelqu’un = to call someone

So:

• Tu peux appeler Marie. = You can call Marie.

This is different from some other French verbs that do need a preposition. For example:

• parler à Marie = to speak to Marie

But with appeler, you say:

• appeler Marie
• not appeler à Marie ✘

They are close, but the grammar is different.

• appeler quelqu’un = to call someone
• téléphoner à quelqu’un = to telephone someone / call someone

So you can say:

• Tu peux appeler Marie.
• Tu peux téléphoner à Marie.

Both can mean You can call Marie, but the structure changes:

• appeler Marie
• téléphoner à Marie

Yes. In everyday spoken French, many speakers drop ne.

So instead of:

• Je ne suis pas disponible avant midi

you will often hear:

• Je suis pas disponible avant midi

This is very common in conversation, but in careful writing and standard French, keep ne.

The main change would be tu to vous and the verb would change too:

• Je ne suis pas disponible avant midi, mais vous pouvez appeler Marie.

That means:

• I’m not available before noon, but you can call Marie.

So the informal part is:

• tu peux

and the formal/plural version is:

• vous pouvez

Because Marie is the direct object of the verb appeler, and in French, direct objects usually come after the verb unless they are replaced by a pronoun.

So:

• appeler Marie = call Marie

If you replace Marie with a pronoun, the pronoun comes before the verb:

• Tu peux l’appeler. = You can call her.

That word order can feel unusual to English speakers, so it is worth noticing.

A natural pronunciation is approximately:

Je ne suis pas disponible avant midi, mais tu peux appeler Marie.
→ zhuh nuh swee pah dee-spoh-neebl ah-vahn mee-dee, meh too puh ah-play mah-ree

A few useful pronunciation points:

• Je sounds like zhuh
• suis sounds like swee
• the s in pas is normally silent
• disponible ends with an audible -ble
• mais sounds like meh
• peux sounds roughly like puh
• appeler sounds like ah-play

Not quite.

• Je ne suis pas disponible avant midi = I’m not available before noon
• Je ne suis pas disponible jusqu’à midi = I’m not available until noon

In this context, they are very close in meaning, since both suggest availability starts at noon. But the phrasing is slightly different:

• avant midi focuses on the time before noon
• jusqu’à midi focuses on the period up to noon

Both are natural, but the sentence you have uses avant midi.