Metto l’auto nel garage.

Mettere is the infinitive, meaning to put.
Metto is the first-person singular present tense form, meaning I put or I am putting.

So:

• mettere = to put
• metto = I put

Italian verbs change form depending on the subject.

Italian often leaves out subject pronouns when they are clear from the verb ending.

Because metto already means I put, you do not need io.

So both are possible:

• Metto l’auto nel garage.
• Io metto l’auto nel garage.

The version without io is more natural unless you want emphasis, such as I’m the one putting the car in the garage.

L’auto means the car.

The full feminine singular article is la, but before a word starting with a vowel, la becomes l’.

So:

• la macchina
• l’auto

The apostrophe shows that a vowel has been dropped.

In Italian, when la comes before a vowel, it is shortened to l’ with an apostrophe.

So you write:

• la + auto → l’auto

You do not combine them into one word without the apostrophe.

Both can mean car.

• auto = car, automobile
• macchina = car, but also machine

In everyday Italian, macchina is extremely common for car.
Auto is also correct and common, and may sound a little more neutral or formal depending on context.

So this sentence could also be:

• Metto la macchina nel garage.

Nel means in the.

It is a contraction of:

• in + il = nel

This kind of combination is very common in Italian.

Other examples:

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

Because garage here is a specific noun with an article: the garage.

So:

• nel garage = in the garage

Italian often uses articles where English may or may not.
If you said just in garage, it would generally sound incomplete or unnatural in standard Italian.

Italian often uses in where English uses in or into, depending on context.

Here, the idea of movement is already suggested by metto (I put), so nel garage naturally means putting something into the garage.

Italian does not always need a special form corresponding exactly to English into.

In this sentence, garage is treated as masculine singular, which is why you get:

• il garage
• nel garage

This is common with many borrowed nouns in Italian.

The verb form metto is present tense.

It can mean:

• I put
• I’m putting

Italian present tense often covers both the simple present and the present progressive, depending on context.

Not exactly in the most natural sense.

Mettere l’auto nel garage literally means to put the car in the garage. Depending on context, that could indeed refer to parking it there, but if you specifically want to say I park the car in the garage, Italian might more naturally use:

• Parcheggio l’auto nel garage.

Still, the original sentence is perfectly understandable.

This is the normal Italian word order:

• verb + object + place

So:

• Metto = I put
• l’auto = the car
• nel garage = in the garage

Italian word order is often similar to English, though it can be changed for emphasis.

Yes, that is grammatically possible, but it is less neutral.

• Metto l’auto nel garage = standard, neutral order
• Metto nel garage l’auto = more marked, with extra focus on l’auto or on the location

Learners should usually stick with Metto l’auto nel garage unless they have a reason to change the emphasis.

Yes, mettere is not completely regular.

Its present tense forms are:

• metto = I put
• metti = you put
• mette = he/she/it puts
• mettiamo = we put
• mettete = you all put
• mettono = they put

A learner often notices the double tt, which stays in these forms.

You put non before the verb:

• Non metto l’auto nel garage.

That means I do not put / I am not putting the car in the garage.

In Italian, you usually keep the same word order and show the question through intonation in speech or a question mark in writing:

• Metto l’auto nel garage?

This means Am I putting the car in the garage? or Do I put the car in the garage?, depending on context.

Yes. The direct object pronoun for l’auto is la, because auto is feminine.

So:

• Metto l’auto nel garage.
• La metto nel garage. = I put it in the garage.

Notice that the pronoun usually comes before the verb.

Many borrowed words in Italian, especially modern ones, often stay unchanged in form.

So:

• il garage
• i garage

The singular and plural can look the same, and the article tells you the number.