Il maiale dorme nel fieno e il topo sparisce sotto la stalla.

Because Italian normally uses an article with nouns like these. Here, both animals are singular masculine nouns, so they take il:

• il maiale
• il topo

Italian also normally repeats the article before the second noun, so il maiale ... e il topo ... sounds natural. English sometimes drops repeated words more easily, but Italian usually keeps the article.

That is a very common question. In Italian:

• many masculine nouns end in -o
• many feminine nouns end in -a
• nouns ending in -e can be either masculine or feminine

So maiale happens to be masculine, and you learn that from the article: il maiale. The ending -e does not tell you the gender by itself.

Because dormire and sparire are the infinitives: to sleep and to disappear.

In the sentence, the verbs are conjugated for:

• present tense
• third person singular
• because the subjects are singular: il maiale, il topo

So:

• dormire → dorme
• sparire → sparisce

This is the equivalent of English sleeps and disappears.

Because not all -ire verbs behave the same way in the present tense.

Some -ire verbs use an extra -isc- in some forms:

• sparire → sparisco, sparisci, sparisce, spariscono

But dormire does not use that pattern:

• dormire → dormo, dormi, dorme, dormono

So this difference is something you have to learn with each verb. A good rule is: memorize the present tense pattern when you learn a new -ire verb.

Nel is a contraction of:

• in + il = nel

So nel fieno literally means in the hay.

Italian often combines certain prepositions with definite articles:

• in + il = nel
• di + il = del
• a + il = al
• da + il = dal
• su + il = sul

Because in commonly combines with the article, but sotto normally stays separate.

So you get:

• in + il fieno → nel fieno
• sotto + la stalla → sotto la stalla

Also, the meanings are different:

• nel fieno = in the hay
• sotto la stalla = under the stable/barn

So this is both a grammar difference and a meaning difference.

Because stalla is a feminine singular noun. For most feminine singular nouns beginning with a consonant, Italian uses la.

So:

• la stalla

This is different from the masculine nouns in the sentence:

• il maiale
• il topo
• il fieno (inside nel fieno)

Often, the ending gives you a clue:

• nouns in -o are often masculine → fieno
• nouns in -a are often feminine → stalla

So here:

• fieno → masculine → il fieno, nel fieno
• stalla → feminine → la stalla

But these are patterns, not absolute rules. The safest habit is to learn each noun together with its article.

Italian word order is more flexible than English, although this sentence uses a very normal, neutral order:

• Il maiale dorme nel fieno e il topo sparisce sotto la stalla.

You could change the order for emphasis, for example:

• Nel fieno dorme il maiale.
• Sotto la stalla sparisce il topo.

These are possible, but they sound more marked or literary. For a learner, the original subject + verb + place order is the safest and most natural choice.

Normally, no. You would say:

• Il maiale dorme nel fieno e il topo sparisce sotto la stalla.

Italian usually needs the article again before the new noun. Leaving it out would sound incomplete or ungrammatical in this sentence.

Most naturally, it sounds like it is talking about a specific pig and a specific mouse, especially in a simple descriptive sentence.

However, Italian often uses the definite article in places where English might use a more general form. So depending on context, il maiale and il topo can sometimes feel a little broader than English the pig and the mouse.

In this sentence alone, though, the simplest interpretation is just a straightforward description of two particular animals.