Nel sacchetto blu metto solo il cibo surgelato.

Nel is a contraction of in + il, so it means in the.

• in = in / into
• il = the
• nel = in the

Here, Nel sacchetto blu means in the blue bag or into the blue bag, depending on context. With verbs like mettere (to put), English often prefers into, even though Italian still uses in.

Sacchetto means small bag, bag, or sack, often something like a shopping bag, plastic bag, or paper bag.

It is not exactly the same as borsa:

• sacchetto = bag/sack, often practical or disposable
• borsa = handbag, purse, or a larger bag

So sacchetto blu is specifically the blue bag in the sense of a bag used for carrying or sorting things.

In Italian, color adjectives usually come after the noun.

So:

• il sacchetto blu = the blue bag
• literally: the bag blue

This is very normal in Italian. English speakers often expect the adjective before the noun, but Italian usually puts descriptive adjectives like colors after it.

Blu is one of the Italian color words that is typically invariable, meaning it does not change form.

So you get:

• il sacchetto blu
• la borsa blu
• i sacchetti blu
• le borse blu

The word blu stays the same in all these cases.

Italian often leaves out subject pronouns when they are already clear from the verb form.

• metto = I put
• io metto = I put

Because the ending -o already shows the subject is I, the pronoun io is usually unnecessary.

Italian does this much more often than English. You only add io for emphasis, contrast, or clarity.

In this sentence, solo means only, and it most naturally modifies il cibo surgelato.

So the idea is:

I put only frozen food in the blue bag.

That means the restriction is on what is being put in the bag.

Italian word order can shift for emphasis, but here solo il cibo surgelato clearly goes together.

Yes, Italian can move solo around, but the emphasis may change slightly.

For example:

• Nel sacchetto blu metto solo il cibo surgelato.
  = I put only frozen food in the blue bag.

• Metto solo il cibo surgelato nel sacchetto blu.
  = same basic meaning, with a slightly different flow

• Solo nel sacchetto blu metto il cibo surgelato.
  = Only in the blue bag do I put frozen food.
  Here the restriction is on the bag, not on the food.

So placement matters because it can change what only applies to.

Italian often uses the definite article more than English does.

Here, il cibo surgelato means the frozen food, but in natural English we might simply say frozen food.

Italian frequently includes the article with nouns in contexts where English would leave it out. So this is normal and not something you should translate too literally every time.

Both relate to freezing, but they are not always exactly the same.

• surgelato usually means deep-frozen or commercially frozen food
• congelato means frozen more generally

In everyday contexts involving food from the supermarket, cibo surgelato is very natural for frozen food.

So in this sentence, surgelato strongly suggests food kept frozen as a product category.

It can correspond to either in English, depending on how natural the English sentence is.

With mettere (to put), English often prefers into because there is movement:

• I put only frozen food into the blue bag

But English also often says:

• I put only frozen food in the blue bag

Italian uses in in both kinds of situations, so nel covers what English might express as either in the or into the.

Yes, this word order is natural.

Nel sacchetto blu metto solo il cibo surgelato is a perfectly normal sentence. It starts with the location, which gives it a slight topical emphasis, as if the speaker is saying:

As for the blue bag, I put only frozen food in it.

A more neutral order could also be:

Metto solo il cibo surgelato nel sacchetto blu.

Both are correct. The original sentence just highlights the blue bag first.