La vela e l'albero maestro sono pronti.

Because Italian usually elides the article lo/la or il before certain words beginning with a vowel.

Here, albero begins with a-, so il albero becomes l'albero.

• il libro
• l'albero
• l'amico
• l'isola

The apostrophe shows that a vowel has been dropped.

Because the subject is compound: La vela e l'albero maestro = the sail and the mast.

That means there are two things, so the verb must be plural:

• La vela è pronta. = The sail is ready.
• L'albero maestro è pronto. = The mast is ready.
• La vela e l'albero maestro sono pronti. = The sail and the mast are ready.

So sono is the third-person plural of essere.

Because pronti is the masculine plural form of pronto.

In Italian, when an adjective refers to a mixed group of masculine and feminine nouns, the adjective normally takes the masculine plural.

Here:

• vela is feminine singular
• albero maestro is masculine singular

Together, they form a mixed group, so the adjective becomes pronti.

Compare:

• La vela è pronta.
• Le vele sono pronte.
• L'albero maestro è pronto.
• Gli alberi maestri sono pronti.
• La vela e l'albero maestro sono pronti.

Yes, vela is feminine.

One clue is the article:

• la vela

Also, many nouns ending in -a are feminine, though not all. In this sentence, you can clearly see it is feminine because if it were alone, the adjective would be feminine too:

• La vela è pronta.

So the article la and the adjective form help confirm the gender.

It is a noun + adjective combination that functions as one unit.

• albero = mast
• maestro = main / principal

So albero maestro means main mast.

Even though maestro often means teacher or master in other contexts, here it is an adjective meaning something like main, principal, or chief.

In nautical language, albero maestro is a fixed expression.

Because maestro belongs only to albero, not to both nouns.

The sentence structure is:

• La vela
• e
• l'albero maestro
• sono pronti

So maestro is part of the noun phrase l'albero maestro. It describes albero only.

The plural adjective that describes the whole subject is pronti.

Sometimes Italian adjectives can come before the noun, but in this sentence pronti is not directly before or after one noun. It comes after the verb because it is a predicate adjective.

Structure:

• La vela e l'albero maestro = subject
• sono = verb
• pronti = adjective describing the subject

This is very normal in Italian, just like:

• Il caffè è caldo.
• Le ragazze sono stanche.

So pronti is placed after sono, not before the nouns.

Because albero maestro is one noun phrase.

The article l' belongs to the noun albero, and maestro modifies that noun. Italian does not normally repeat the article inside this kind of phrase.

So:

• l'albero maestro = the main mast

not something like:

• l'albero il maestro ✘

In theory, e can become ed before a word beginning with a vowel, especially e itself, for smoother pronunciation.

But in modern Italian, this is much less common than learners sometimes expect. In your sentence, e l'albero is completely normal.

So:

• La vela e l'albero maestro ✔

Using ed here would sound formal or old-fashioned to many speakers, and it is not the usual choice.

Because when two nouns share the same description, Italian often uses one plural adjective after the verb.

So instead of saying:

• La vela è pronta e l'albero maestro è pronto

Italian can combine them into:

• La vela e l'albero maestro sono pronti.

This is more natural and efficient when both items share the same quality.

Pronto/pronta/pronti/pronte can mean ready, prepared, or set depending on context.

In this sentence, it means that the sail and the main mast are ready. The exact nuance depends on the situation: ready for sailing, ready for use, properly set up, etc.

It is a very common adjective in Italian.

Then the adjective would become feminine plural.

For example:

• La vela e la corda sono pronte.

Because both vela and corda are feminine, the adjective agrees in the feminine plural: pronte.

So the masculine plural pronti in your sentence is specifically because the group includes the masculine noun albero.

The verb and adjective would both become singular and agree with that one noun.

Examples:

• La vela è pronta.
• L'albero maestro è pronto.

So with one feminine noun: è pronta
With one masculine noun: è pronto
With both together: sono pronti