Più stringi le viti, più è facile montare la mensola.

This is a very common Italian pattern that corresponds to English the more ..., the more ... or the more ..., the easier ....

So:

• Più stringi le viti = The more you tighten the screws
• più è facile montare la mensola = the easier it is to install/assemble the shelf

In Italian, this repeated più creates a comparison of increasing degree: as one thing increases, another changes too.

A very similar example is:

• Più studi, più impari.
  The more you study, the more you learn.

Stringi is the present indicative, second person singular of stringere.

• stringere = the infinitive, to tighten
• stringi = you tighten

Italian often uses the generic you in instructions, observations, and general truths, just like English sometimes does:

• Più mangi, più ingrassi.
• The more you eat, the more you gain weight.

You could say stringi tu, but that would sound marked or emphatic, as if you were specifically contrasting you with someone else. Normally, the pronoun is omitted.

Because Italian usually does not need subject pronouns when the verb ending already shows who the subject is.

In stringi, the ending -i tells us the subject is tu.

So Italian normally says:

• Stringi le viti = You tighten the screws

not:

• Tu stringi le viti

unless you want emphasis, contrast, or clarity.

This is one of the big differences from English, where the pronoun is usually required.

Here stringere means to tighten.

That verb has a broader meaning in Italian and can also mean things like:

• to squeeze
• to clutch
• to grip
• to tighten

With viti (screws), the natural meaning is to tighten:

• stringere una vite = to tighten a screw

The opposite would often be allentare = to loosen.

Italian uses definite articles much more often than English.

So le viti literally means the screws, but in many contexts English would simply say screws.

Italian often prefers the article with plural nouns, body parts, everyday objects, and things understood from context.

Here, le viti refers to the screws involved in assembling the shelf, so the article sounds natural.

Both are possible, but they are structured differently.

È facile montare la mensola
literally: It is easy to assemble the shelf

This is an impersonal structure:

• è facile = it is easy
• montare la mensola = infinitive phrase acting like the real subject

You could also say:

• La mensola è facile da montare.
  The shelf is easy to assemble.

That version focuses more directly on the shelf as the subject.

In your sentence, è facile montare la mensola works well because it fits naturally after the più ..., più ... pattern.

Because after expressions like è facile, è difficile, è importante, è utile, Italian commonly uses an infinitive verb.

So:

• È facile montare la mensola = It is easy to assemble the shelf
• È difficile capire = It is difficult to understand
• È utile sapere questo = It is useful to know this

This is very similar to English it is easy to + verb.

The sentence uses the present indicative:

• stringi = present
• è = present

Italian often uses the present tense for:

• general truths
• instructions
• habitual situations
• cause-and-effect statements

So this sentence is not necessarily talking about one specific moment only. It expresses a general principle:

• when you tighten the screws more, assembling the shelf becomes easier

English does the same thing:

• The more you tighten the screws, the easier it is to assemble the shelf.

The comma is very normal here and helps separate the two parts of the correlative structure:

• Più stringi le viti, più è facile montare la mensola.

It makes the sentence easier to read and reflects the pause you would naturally make.

In practice, with this kind of più ..., più ... construction, a comma is commonly used.

Because più è facile montare la mensola follows normal modern Italian word order.

The core part is:

• è facile montare la mensola = it is easy to assemble the shelf

Then più modifies the idea of degree:

• più è facile = the easier it is

You might occasionally see more literary or marked word order in Italian, but più è facile is the natural standard form here.

Yes, that is possible, but it is not exactly the same in focus.

• Più stringi le viti, più è facile montare la mensola
  = The more you tighten the screws, the easier it is to assemble the shelf.

This focuses on the degree of ease.

• Più stringi le viti, più facilmente monti la mensola
  = The more you tighten the screws, the more easily you assemble the shelf.

This version focuses more directly on how easily you perform the action.

Both are grammatical. Your original sentence sounds slightly more neutral and natural as a general statement.

Mensola usually means shelf.

Depending on context, montare la mensola means something like:

• to put up the shelf
• to install the shelf
• to assemble the shelf

If the sentence is about furniture assembly or wall mounting, shelf is the natural meaning.

Yes. A more formal or literary alternative is:

• Quanto più stringi le viti, tanto più è facile montare la mensola.

This also means:

• The more you tighten the screws, the easier it is to assemble the shelf.

But in everyday modern Italian, the shorter più ..., più ... pattern is very common and natural.