Le mécanicien dit que le pneu avant est encore dégonflé.

In French, you usually need an article before a singular countable noun. So le mécanicien means the mechanic.

French uses articles more often than English does. In English, you might sometimes say something like Mechanic says... in a note or headline, but in normal French you would usually say Le mécanicien dit...

If you wanted a mechanic, you would say un mécanicien.

Dit is the third-person singular form of dire (to say), so it matches le mécanicien:

• je dis = I say
• tu dis = you say
• il / elle dit = he / she says

Que means that and introduces the next clause:

• Le mécanicien dit que... = The mechanic says that...

Just like in English, that can sometimes be omitted in translation, but in French que is normally kept here.

In this sentence, avant is being used as an adjective meaning front. In French, many descriptive adjectives come after the noun, so:

• le pneu avant = the front tire
• le pneu arrière = the rear tire

You may also hear le pneu de devant, which is more literal and a bit more conversational, but le pneu avant is the standard compact way to say it.

So here, avant is not the preposition meaning before. It is an adjective meaning front.

French often uses position words like avant and arrière as adjectives after a noun. So:

• un siège avant = a front seat
• la porte arrière = the rear door
• le pneu avant = the front tire

English often puts front before the noun, but French commonly puts this kind of adjective after it.

Here, dégonflé is best understood as an adjective meaning flat or deflated.

• est = is
• dégonflé = deflated / flat

So est dégonflé means is flat or more literally is deflated.

Although dégonflé comes from the verb dégonfler (to deflate), in this sentence it describes the state of the tire, not an action happening right now.

Yes, it changes to agree with the noun it describes.

Here, pneu is masculine singular, so the form is:

• dégonflé

Other possible forms would be:

• dégonflée for feminine singular
• dégonflés for masculine plural
• dégonflées for feminine plural

Examples:

• La roue est dégonflée.
• Les pneus sont dégonflés.

Here, encore means still.

So est encore dégonflé means is still flat.

That suggests the tire was already flat before, and the problem has not been fixed.

Encore can also mean again in other contexts, which is why learners often find it tricky. The exact meaning depends on context.

Examples:

• Il est encore là. = He is still there.
• Dis-le encore. = Say it again.

In your sentence, still is the natural meaning.

Sometimes, but encore is more natural here.

• encore dégonflé = still flat, still deflated
• toujours dégonflé can also mean still flat, but it can sound more like always flat depending on context

Because the idea is that the tire remains in that condition after someone expected it to be fixed, encore fits very well.

French sentence structure here is very straightforward:

• Le mécanicien = subject
• dit = verb
• que... = introduces a subordinate clause
• le pneu avant = subject of the subordinate clause
• est = verb
• encore dégonflé = complement

So it is literally:

• The mechanic says that the front tire is still flat.

French does not need an extra word like does here. English uses do/does in some structures, but French does not.

A careful pronunciation would be roughly:

Luh may-ka-nee-syen dee kuh luh pnu ah-vahn ehz ahn-kor day-gohn-flay

A few useful notes:

• mécanicien ends with a nasal sound in -ien
• pneu is a tricky one-syllable word
• avant has a nasal an sound
• est encore may sound linked in natural speech

A more IPA-style pronunciation is approximately:

lə me.ka.ni.sjɛ̃ di kə lə pnø a.vɑ̃ ɛ.t‿ɑ̃.kɔʁ de.ɡɔ̃.fle

Yes. That depends on whether you use American or British English.

• pneu = tire in American English
• pneu = tyre in British English

So both are correct English equivalents.