Fais attention à cette flaque de boue : tu peux glisser même avec de bonnes chaussures.

Because fais is a command here.

French commands use the imperative, and in the imperative the subject pronoun is usually omitted:

• Fais attention = Be careful / Watch out
• Faites attention = Be careful / Watch out for you all or you in a polite/formal sense

So Fais attention is not a statement like You are being careful. It is an instruction directed at tu.

Faire attention à is a very common expression meaning:

• to be careful of
• to watch out for
• to pay attention to

In this sentence, Fais attention à cette flaque de boue means something like Watch out for that muddy puddle.

The à is important when you mention the thing you should watch out for:

• Fais attention ! = Be careful!
• Fais attention à la marche. = Watch out for the step.
• Fais attention à cette flaque. = Watch out for that puddle.

Because à and a are two different words.

• à = a preposition, often meaning to, at, in
• a = the verb has from avoir

So here:

• attention à cette flaque = attention to / about that puddle

Not:

• a = has

The accent helps you tell them apart in writing.

Because flaque is a feminine singular noun.

French demonstratives must agree with the noun:

• ce for masculine singular
• cet for masculine singular before a vowel sound
• cette for feminine singular
• ces for plural

Since une flaque is feminine, you need cette flaque.

Literally, it means puddle of mud.

• flaque = puddle
• boue = mud
• de links them together

In natural English, you might say:

• a puddle of mud
• a muddy puddle
• a mud puddle

All fit the idea.

Because after pouvoir (can / to be able to), the next verb stays in the infinitive.

So:

• tu peux glisser = you can slip

Not:

• tu peux glisses

This is the same pattern as:

• tu peux venir = you can come
• tu peux partir = you can leave
• tu peux tomber = you can fall

It can mean both, depending on context.

• glisser = to slide
• glisser = to slip

In this sentence, because of the muddy puddle, slip is the best translation:

• tu peux glisser = you could slip

So the idea is that the puddle is slippery and dangerous.

Même avec means even with.

So:

• tu peux glisser même avec de bonnes chaussures
• you can slip even with good shoes

It adds emphasis. The speaker is saying that good shoes do not completely protect you. The puddle is still risky.

This is a classic French grammar point.

Normally, the plural indefinite article is des:

• des chaussures = some shoes

But in standard French, des often becomes de when an adjective comes before the noun:

• des chaussures
• de bonnes chaussures

So de bonnes chaussures is the standard written form.

You may sometimes hear des bonnes chaussures in casual speech, but learners should usually prefer de bonnes chaussures in careful French.

Because bon has to agree with chaussures, which is feminine plural.

Forms of bon:

• bon = masculine singular
• bonne = feminine singular
• bons = masculine plural
• bonnes = feminine plural

Since chaussures is feminine plural, the correct form is bonnes.

Because the sentence is addressing one person informally, or it may be using tu in a general, everyday way.

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

If you wanted the polite or plural version, you could say:

• Faites attention à cette flaque de boue : vous pouvez glisser même avec de bonnes chaussures.

So the original sentence sounds familiar and direct, like something you would say to a friend, child, or family member.