Faccio retromarcia lentamente perché il garage è stretto.

In Italian, the subject pronoun is often omitted because the verb ending already tells you who is doing the action.

• faccio = I do / I am doing
• The ending -o shows it is I

So Faccio retromarcia lentamente is perfectly natural. You could say Io faccio retromarcia lentamente if you want extra emphasis, but normally io is not necessary.

Yes, it is a common and natural expression.

Literally:

• faccio = I do / I make
• retromarcia = reverse gear / reversing

So fare retromarcia means to reverse / to back up a vehicle.

It works like an idiomatic expression: Italian often uses fare + noun where English might use a simple verb.

Examples:

• Faccio retromarcia. = I’m reversing.
• Ha fatto retromarcia. = He/She reversed.

Italian often uses the simple present where English uses either the simple present or the present continuous.

So:

• Faccio retromarcia can mean I reverse or I’m reversing, depending on context.

If you want to be more explicitly progressive, Italian can also use stare + gerundio:

• Sto facendo retromarcia.

But in many everyday situations, the simple present is the most natural choice.

It is very often used with fare in this kind of sentence:

• fare retromarcia = to reverse

You can also see retromarcia in other expressions, for example:

• mettere in retromarcia = to put into reverse
• la retromarcia = reverse gear

So retromarcia is a noun, and fare retromarcia is the full verbal expression.

Because it describes how the action is done, so Italian uses an adverb.

• lento = slow (adjective)
• lentamente = slowly (adverb)

Here it modifies the action faccio retromarcia:

• Faccio retromarcia lentamente. = I reverse slowly.

Compare:

• una macchina lenta = a slow car
• guida lentamente = he/she drives slowly

They often come after the verb or after the verbal expression.

So this sentence is very natural:

• Faccio retromarcia lentamente.

Italian word order is somewhat flexible, but this position is common and clear. You might also hear different placements for emphasis, but the standard order here is very normal.

Perché has an accent because it is stressed on the final -é.

It can mean:

• because
• why (in a question)

In your sentence it means because:

• ...perché il garage è stretto. = ...because the garage is narrow.

The accent is important in standard spelling.

Yes, garage is normally treated as a masculine noun in Italian, so it commonly takes il:

• il garage

Also, garage is usually invariable, meaning it often keeps the same form in the plural:

• il garage
• i garage

Loanwords in Italian often behave this way.

Because stretto agrees with il garage, which is masculine singular.

Agreement works like this:

• masculine singular: stretto
• feminine singular: stretta
• masculine plural: stretti
• feminine plural: strette

Since garage is masculine singular, Italian uses:

• il garage è stretto

It can mean several related things depending on context, including:

• narrow
• tight
• close
• restricted

In this sentence, stretto means that the garage has little space, so narrow or tight is the right idea.

Examples:

• una strada stretta = a narrow road
• jeans stretti = tight jeans

Because retromarcia is part of the fixed expression fare retromarcia.

Italian often uses a bare noun in expressions like this, without an article. So:

• fare retromarcia = to reverse
• not usually fare la retromarcia in this meaning

Even though retromarcia is a noun, in this expression it behaves like part of the verbal phrase.

It can be either, depending on context.

The Italian present tense is flexible:

• Faccio retromarcia lentamente perché il garage è stretto. can mean
  I reverse slowly because the garage is narrow
  or
  I’m reversing slowly because the garage is narrow

If the speaker is talking about what they are doing at the moment, the second interpretation is natural. If they are describing a general habit, the first interpretation works.

Yes, that is grammatically correct and it emphasizes that the action is happening right now.

• Faccio retromarcia... = neutral, very common
• Sto facendo retromarcia... = explicitly I am reversing...

However, in ordinary spoken Italian, the simple present is often preferred unless you specifically want to stress the ongoing nature of the action.

Not exactly. In Italian, the pronunciation is usually adapted to Italian phonology, and it does not sound exactly like English garage.

Learners mainly need to know that:

• it is a borrowed word
• it is common in everyday Italian
• it normally takes il

In speech, pronunciation can vary somewhat depending on speaker and region, but the word is fully normal in Italian usage.