Mets ton clignotant avant le virage, surtout quand il faut freiner près de l’école.

Mets is the tu imperative form of mettre.

In French, when you give a command to someone directly, you usually drop the subject pronoun:

• Tu mets ton clignotant. = You put on your indicator.
• Mets ton clignotant. = Put on your indicator.

So mets here is a command addressed to one person informally.

Because mettre is not an -er verb.

In the tu imperative, many -er verbs lose the final -s:

• Regarde !
• Tourne !

But verbs like mettre, prendre, sortir, etc. usually keep it:

• Mets !
• Prends !
• Sors !

So Mets is the correct imperative form.

Yes, it is completely normal.

French often says mettre son clignotant, literally to put on one’s indicator/blinker, where English might say:

• put your indicator on
• use your turn signal
• signal

So Mets ton clignotant is a very natural driving instruction.

You may also hear:

• mettre le clignotant
• mettre son clignotant à gauche / à droite

Because clignotant is a masculine singular noun, so the possessive adjective must match the noun:

• mon / ton / son
  • masculine singular noun
• ma / ta / sa
  • feminine singular noun
• mes / tes / ses
  • plural noun

So:

• ton clignotant
• but ta voiture

The possessive agrees with the thing owned, not with the gender of the person being spoken to.

Here, le virage means the bend / the turn in the road.

That is slightly different from the action to turn, which is a verb:

• tourner = to turn
• un virage = a bend, curve, or turn in the road

So avant le virage means before the bend/turn, not necessarily before turning yourself in a more general sense.

Because avant is followed by de only before a verb, but before a noun you normally use avant + noun directly.

Compare:

• avant le virage = before the turn
• avant de tourner = before turning

So with the noun le virage, the correct form is simply avant le virage.

Surtout means especially or above all.

It adds emphasis:

• Mets ton clignotant avant le virage = Put on your indicator before the turn.
• surtout quand il faut freiner près de l’école = especially when you have to brake near the school.

So the speaker is saying this is important in general, but even more important in that situation.

Il faut is a very common impersonal expression meaning:

• it is necessary
• you have to
• one must

So quand il faut freiner means when you have to brake.

Important point: the il here does not refer to a person. It is just part of the fixed expression il faut.

Examples:

• Il faut partir. = We/you/one must leave.
• Il faut freiner. = You have to brake.

Because falloir (used almost always as il faut) is followed by an infinitive when the action is general.

So:

• il faut freiner = it is necessary to brake / you have to brake

This is similar to English have to + verb.

Other examples:

• Il faut attendre.
• Il faut tourner à gauche.
• Il faut ralentir.

Près de is a fixed expression meaning near or close to.

So:

• près de l’école = near the school

The de is required after près.

Then de + l’école stays de l’école because école begins with a vowel sound. Compare:

• près de la maison
• près du parc
• près de l’école

Because école begins with a vowel, so la école becomes l’école.

This is called elision. French often drops the final vowel of a short word before another vowel sound:

• la école → l’école
• je ai → j’ai
• ne est → n’est

Since école is feminine, the full article is la, but before the vowel it becomes l’.

A learner-friendly pronunciation would be roughly:

May ton klee-nyo-tahn ah-vahn luh vee-rahzh

A few useful points:

• Mets sounds like may
• ton has a nasal vowel; the n is not strongly pronounced
• clignotant has the gn sound like ny in canyon
• virage ends with the soft zh sound, like the s in measure

Also, in natural speech, some sounds may link smoothly together, but for a learner, saying each word clearly is perfectly fine.

Yes. The imperative form Mets corresponds to tu, so it is informal singular.

If you were speaking politely to one person, or to more than one person, you would use vous:

• Mettez votre clignotant avant le virage...

So:

• Mets ton clignotant = informal singular
• Mettez votre clignotant = formal singular or plural

Yes. On its own, clignotant can describe something flashing or blinking, and as a noun it can refer to a blinker/indicator light.

In driving contexts, though, mettre son clignotant very clearly means to use your turn signal / indicator.

So in this sentence, the meaning is unambiguous.