Marta sbaglia meno quando controlla se il sostantivo è al singolare o al plurale.

Literally, sbaglia meno means makes fewer mistakes or is wrong less often.

The verb sbagliare means to make a mistake / to be wrong.
In this sentence, meno means less or fewer.

So:

• Marta sbaglia = Marta makes mistakes / Marta is wrong
• Marta sbaglia meno = Marta makes fewer mistakes / Marta is wrong less often

In natural English, you would often translate it as Marta makes fewer mistakes.

Meno is the normal word for less or fewer in Italian.

So:

• più = more
• meno = less / fewer

You use meno directly with verbs, adjectives, or nouns depending on the structure.

Here it modifies the verb sbaglia, so sbaglia meno means she makes mistakes less often or she makes fewer mistakes.

Italian does not normally say più poco for this meaning.

Italian often leaves out subject pronouns when the verb ending already makes the subject clear. This is called a pro-drop language.

So instead of saying:

• Marta sbaglia meno quando lei controlla...

Italian normally just says:

• Marta sbaglia meno quando controlla...

Because controlla is third person singular, and the subject is clearly still Marta.

English usually needs the pronoun she, but Italian often does not.

Quando means when, but in sentences like this it often has the sense of whenever.

So:

• quando controlla = when she checks / whenever she checks

The sentence is describing a general pattern or habit, not one single event. That is why English might naturally translate it as whenever even though Italian uses quando.

Here se is best understood as whether, even though English sometimes also uses if.

In this sentence:

• controlla se il sostantivo è al singolare o al plurale

means:

• she checks whether the noun is singular or plural

This is different from conditional if.

Compare:

• Se studi, impari. = If you study, you learn.
• Controlla se è giusto. = Check whether it is correct.

So in your sentence, se introduces an indirect yes/no question: Is the noun singular or plural?

They do different jobs.

• quando controlla = when / whenever she checks
• se il sostantivo è al singolare o al plurale = whether the noun is singular or plural

So the structure is:

• Main clause: Marta sbaglia meno
• Time clause: quando controlla
• What she checks: se il sostantivo è al singolare o al plurale

A helpful way to see it is:

• Marta makes fewer mistakes
• when she checks
• whether the noun is singular or plural

So quando tells you when she makes fewer mistakes, and se tells you what she is checking.

Italian often uses the definite article where English might not.

Here il sostantivo means the noun. It refers to the noun being examined in that context.

Italian regularly uses articles with grammatical terms and categories, for example:

• il verbo = the verb
• il sostantivo = the noun
• il singolare = the singular
• il plurale = the plural

Even when English might say something more generally, Italian often keeps the article.

Yes. Sostantivo is a standard Italian word for noun.

You may also see nome used in grammar, especially in school contexts:

• sostantivo
• nome

Both can mean noun, though sostantivo is a bit more technical/formal.

So in this sentence, il sostantivo simply means the noun.

Literally, it means:

• is in the singular or in the plural

In more natural English, that becomes:

• is singular or plural

Italian commonly uses the expression:

• essere al singolare = to be in the singular
• essere al plurale = to be in the plural

So al is part of a normal grammatical expression here.

Al is the contraction of:

• a + il = al

So:

• al singolare literally = to the singular / in the singular
• al plurale literally = to the plural / in the plural

In grammar, Italian uses this pattern idiomatically with essere:

• Il nome è al singolare.
• I nomi sono al plurale.

You do not need to translate a word-for-word here. Just learn essere al singolare / al plurale as a set expression.

Because each part of the alternative keeps the full expression:

• è al singolare
• è al plurale

When they are joined by o (or), Italian normally repeats the preposition + article:

• al singolare o al plurale

This sounds complete and natural. It is the same idea as saying:

• in Italy or in France rather than
• in Italy or France

Italian often prefers repeating the preposition when each item is a full parallel phrase.

Because è is the verb is (from essere), and it must be written with an accent.

This distinguishes it from e, which means and.

So:

• è = is
• e = and

This is a very important spelling difference in Italian.

Because Italian uses the present tense for habitual or general actions, just like English often does.

So this sentence is not necessarily about one single moment. It describes a repeated truth or tendency:

• Marta sbaglia meno = Marta makes fewer mistakes
• quando controlla... = when/whenever she checks...

This is a normal use of the present tense in both languages.

Both, depending on how literal you want to be.

• controlla by itself = checks
• controlla se... = checks whether...

So in this sentence, se completes the meaning of controlla. She is not just checking in a general sense; she is checking whether the noun is singular or plural.

The word order is normal and natural:

• Marta sbaglia meno
• quando controlla
• se il sostantivo è al singolare o al plurale

Italian word order is somewhat flexible, but this version is straightforward and idiomatic.

The sentence starts with the main idea, then adds the circumstance:

• she makes fewer mistakes
• when she checks
• whether the noun is singular or plural

You could rearrange parts in some contexts, but this order is the clearest and most neutral.