In inverno porto un mandarino nello zaino per la merenda, ma a casa preferisco l’uva fresca.

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

• porto already means I carry / I bring / I take
• preferisco already means I prefer

So Io porto... is possible, but it is usually unnecessary unless you want emphasis or contrast.

For example:

• Porto un mandarino... = I carry a mandarin...
• Io porto un mandarino, non tu. = I carry a mandarin, not you.

Porto is from portare. In this sentence it means something like:

• I carry
• I take with me
• I bring

The exact English translation depends on context. Since the sentence talks about having the fruit nello zaino, I carry or I take is very natural.

In inverno means in winter. This is a common way to talk about seasons in a general sense.

Italian often uses:

• in primavera
• in estate
• in autunno
• in inverno

Here it means during the winter season in general, not one specific winter day.

Because in + lo contracts to nello.

So:

• in + lo = nello

You use lo with masculine singular nouns that begin with certain sounds, including z. Since zaino starts with z, you get:

• lo zaino
• nello zaino

Compare:

• nel libro = in the book
• nella borsa = in the bag
• nello zaino = in the backpack

Literally, it means in the backpack, but in natural English we often translate it as in my backpack if the ownership is obvious from context.

Italian very often uses the definite article where English prefers a possessive.

So in this sentence, nello zaino naturally suggests in my backpack, even though my is not stated.

It means for snack, for the snack break, or more naturally in English, for my snack or as a snack.

Merenda is a snack, often an afternoon snack, especially in everyday Italian.

So:

• porto un mandarino... per la merenda
  = I take a mandarin... for my snack

Merenda is a feminine singular noun, so its article is la.

• la merenda = the snack

In this sentence, per la merenda refers to the snack time / snack occasion in a general everyday sense.

A casa is a fixed, very common expression meaning:

• at home
• home

Italian normally says a casa, without an article, when talking about being at home or going home.

Examples:

• Sono a casa. = I am at home.
• Vado a casa. = I’m going home.

Alla casa would usually mean to the house, referring to a specific house as a building, which is different.

They are similar, but not always identical.

• a casa = at home / home
• in casa = in the house / indoors / inside the home

In this sentence, a casa is the natural choice because it means when I’m at home.

So:

• ma a casa preferisco... = but at home I prefer...

Because uva in Italian is usually a singular collective noun. It refers to grapes as a type of fruit or as a mass of grapes.

So:

• l’uva literally looks singular
• but in English it is often translated as grapes

That is why Italian says:

• Preferisco l’uva fresca
  = I prefer fresh grapes

If you wanted to talk about individual grapes, you would usually need a different expression, such as acini d’uva.

Because fresca agrees with uva, and uva is feminine singular.

• uva = feminine singular
• fresca = feminine singular adjective

Even though English uses the plural word grapes, Italian is treating uva as a singular noun, so the adjective must also be singular.

In Italian, adjectives often come after the noun.

So:

• uva fresca = fresh grapes
• pane caldo = warm bread
• acqua fredda = cold water

Putting the adjective after the noun is the most neutral and common pattern here.
If you moved it before the noun, it could sound more marked, poetic, or slightly different in emphasis.

Mandarino usually means mandarin or mandarin orange.

Depending on region and everyday usage, people may sometimes use related fruit words a bit loosely, but for a learner, mandarin is the safest meaning here.

So:

• un mandarino = a mandarin

Because they are being treated differently:

• un mandarino = one countable fruit
• l’uva = grapes in general, treated as a singular mass/collective noun

So the sentence contrasts:

• one individual mandarin for a snack
• grapes in general as the fruit preferred at home

Because preferire belongs to a group of -ire verbs that add -isc- in some present-tense forms.

So:

• io preferisco
• tu preferisci
• lui/lei preferisce
• noi preferiamo
• voi preferite
• loro preferiscono

This pattern is very common with some -ire verbs, such as:

• capire → capisco
• finire → finisco
• preferire → preferisco

Yes. Italian uses the present tense very often for habitual or regular actions.

So this sentence means something like:

• In winter, I usually carry a mandarin in my backpack for snack, but at home I prefer fresh grapes.

It is not necessarily describing only what is happening right now. It can describe a routine or general preference.

Yes, Italian word order is somewhat flexible, but the original sentence is very natural.

For example, you could say:

• In inverno porto nello zaino un mandarino per la merenda...
• Ma a casa preferisco l’uva fresca.
• A casa, però, preferisco l’uva fresca.

The original order is clear and idiomatic, so it is a good model to learn from.