Controllo lo specchietto prima di fare retromarcia nel garage.

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

• controllo = I check / I am checking
• The -o ending tells you the subject is io.

So Controllo lo specchietto is perfectly natural, and adding io would usually only be for emphasis:

• Io controllo lo specchietto, non tu. = I’m the one checking the mirror, not you.

Yes. Controllo is the present indicative of controllare.

In Italian, the present tense often covers both:

• I check
• I’m checking

Which meaning is intended depends on context.

So this sentence could mean:

• a habitual action: I check the mirror before reversing into the garage
• or something happening now: I’m checking the mirror before reversing into the garage

Italian uses articles much more often than English does. Here lo specchietto means the mirror.

In many everyday situations, Italian prefers the definite article where English might use:

• the
• my
• or no article at all

So in a driving context, Controllo lo specchietto is natural for I check the mirror.

Also, lo is used because specchietto begins with sp-, and nouns starting with s + consonant usually take:

• lo in the singular
• gli in the plural

For example:

• lo specchio
• lo sportello
• gli specchietti

Specchietto is the diminutive of specchio (mirror), so literally it is something like little mirror. In everyday Italian, it often refers to a car mirror, especially:

• the side mirror
• or sometimes the rear-view mirror, depending on context

In this sentence, because we are talking about driving and reversing, specchietto clearly refers to a car mirror.

Prima di is followed by an infinitive when the subject is the same in both parts of the sentence.

Here the subject is I throughout:

• Controllo lo specchietto = I check the mirror
• prima di fare retromarcia = before reversing

So prima di + infinitive is exactly what Italian uses.

Compare:

• Controllo lo specchietto prima di fare retromarcia.
  I check the mirror before reversing.

But if the subject changes, Italian normally uses prima che + subjunctive:

• Controllo lo specchietto prima che tu faccia retromarcia.
  I check the mirror before you reverse.

Yes, retromarcia is a noun, and fare retromarcia is a fixed expression meaning:

• to reverse
• to back up

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

So:

• fare retromarcia = to reverse
• literally something like to do/make reverse gear, but you should learn it as a set phrase

You may also hear:

• andare in retromarcia = to go in reverse
• fare marcia indietro = to back up / go backward

Nel is a contraction of:

• in + il = nel

So nel garage means:

• in the garage
• or sometimes into the garage, depending on context

Because the sentence is about reversing, English might naturally say into the garage, even though Italian uses nel garage.

Other similar contractions:

• in + lo = nello
• in + la = nella
• in + i = nei
• in + gli = negli
• in + le = nelle

Italian usually uses an article with a noun like garage, so nel garage is the normal form for in the garage.

English sometimes drops the article in certain expressions, but Italian often keeps it. So:

• nel garage = in the garage
• in casa is possible without an article, but that is a different expression and not the pattern here

Yes, the exact English phrasing depends on context.

nel garage can be understood as:

• in the garage
• into the garage

If the speaker is about to back the car into the garage, English would often say:

• before reversing into the garage

If the car is already in the garage and the speaker is reversing there, then:

• before reversing in the garage

Italian leaves that a bit more open here, and the situation tells you which meaning fits.

This is a very natural Italian word order:

• Controllo = verb
• lo specchietto = object
• prima di fare retromarcia = time/action phrase
• nel garage = place phrase

Italian word order is more flexible than English, but this version sounds neutral and natural.

You could move parts around for emphasis, for example:

• Prima di fare retromarcia nel garage, controllo lo specchietto.

That means the same thing, but it emphasizes before reversing into/in the garage.

Yes, depending on what you want to say.

• lo specchietto = the mirror / one mirror
• gli specchietti = the mirrors

If you mean checking both side mirrors, you could say:

• Controllo gli specchietti prima di fare retromarcia nel garage.

If you mean one particular mirror, singular is fine.

In real speech, both are possible depending on the situation.