Per uscire dal garage devo fare retromarcia piano e lasciare andare la frizione con attenzione.

Because per + infinitive is a very common Italian structure for expressing purpose: in order to ...

So:

Per uscire dal garage = In order to get out of the garage

Here, uscire stays in the infinitive because it comes after per.

You will see this pattern a lot:

• Per imparare l’italiano, devo studiare ogni giorno.
• Per arrivare in tempo, dobbiamo partire presto.

Dal is just the contraction of da + il.

• da + il = dal
• da + lo = dallo
• da + la = dalla
• da + i = dai
• da + gli = dagli
• da + le = dalle

So:

• uscire dal garage
• uscire dalla macchina
• uscire dallo stadio

Italian uses this contraction automatically, just like del, al, nel, and so on.

Because uscire da means to go out of / come out of a place.

So:

• uscire dal garage
• uscire di casa is also possible, but with buildings and locations uscire da is extremely common
• uscire dalla stanza

In this sentence, the idea is movement out from inside the garage, so da is the natural preposition.

Because Italian often leaves out subject pronouns when the verb ending already makes the subject clear.

• devo already means I must / I have to
• so (io) devo is usually just devo

You can include io for emphasis or contrast:

• Io devo fare retromarcia, tu invece puoi andare avanti.

But in a normal sentence, leaving it out is more natural.

Because modal verbs like dovere, potere, and volere are followed directly by the infinitive.

So:

• devo fare
• posso uscire
• voglio andare

Not:

• devo di fare
• devo a fare

Also, in your sentence, devo governs both infinitives:

• devo fare retromarcia
• e (devo) lasciare andare la frizione

The second devo is simply omitted because it is understood.

Fare retromarcia is a very common expression meaning to reverse / to back up a vehicle.

Literally:

• fare = to do / make
• retromarcia = reverse gear / reversing

But together, it functions as a set phrase:

• fare retromarcia = to back up

Italian often uses fare + noun expressions where English uses a simple verb.

Related expressions:

• mettere la macchina in retromarcia = to put the car in reverse
• andare in retromarcia = to go in reverse
• retrocedere = to move backwards / reverse

Because fare retromarcia is a fixed expression, and in this expression retromarcia normally appears without an article.

So:

• fare retromarcia = correct

But if you are talking about the gear itself, you may use an article:

• mettere la retromarcia
• innestare la retromarcia

So the presence or absence of the article depends on the structure being used.

Yes. In this sentence, piano is an adverb meaning slowly, gently, or carefully depending on context.

This is very common in everyday Italian:

• Parla piano. = Speak softly.
• Vai piano. = Go slowly.
• Fai piano! = Be gentle! / Careful!

So fare retromarcia piano means doing the reversing slowly/gently.

A more formal or neutral alternative would be lentamente, but piano is very natural in spoken Italian.

Because Italian adverbs often come after the verb or verb phrase.

So:

• fare retromarcia piano
• guidare lentamente
• parlare piano

That position sounds natural and idiomatic. You could sometimes move adverbs around for emphasis, but in this sentence the placement is completely normal.

Here lasciare andare means something like to let go of gradually or to release.

With la frizione, the idea is:

• let the clutch out
• release the clutch

Italian often uses verb combinations like this where English might use a simple phrasal expression.

In driving contexts, you may also hear:

• rilasciare la frizione = to release the clutch
• lasciare la frizione = let the clutch out
• mollare la frizione = let go of the clutch, more colloquial

So lasciare andare la frizione is understandable as a careful, gradual releasing action.

Because Italian uses definite articles much more often than English.

In English, instructions often sound natural without an article:

• Release clutch
• Turn steering wheel

But in Italian, you normally say:

• rilasciare la frizione
• girare il volante

The article is used because the object is specific and obvious in context: it is the clutch of the car you are driving.

Yes, both can work.

• con attenzione = carefully, with attention
• attentamente = carefully, attentively

In this sentence, con attenzione sounds very natural and conversational. Attentamente is also correct, but it can sound a little more formal or less idiomatic in some everyday driving contexts.

So:

• lasciare andare la frizione con attenzione = very natural
• lasciare andare la frizione attentamente = grammatically correct, but a bit less usual

Yes, it would be fine to write:

Per uscire dal garage, devo fare retromarcia piano e lasciare andare la frizione con attenzione.

The opening phrase Per uscire dal garage is an introductory phrase, so a comma is possible and often helpful in writing.

Without the comma, the sentence is still correct. In short:

• with comma = slightly clearer in writing
• without comma = also perfectly acceptable