Non lasciare che il cane morda il cuscino.

Because this is the negative informal command for tu.

In Italian:

• positive tu command: Lascia! = Let / Leave!
• negative tu command: Non lasciare! = Don’t let / Don’t leave!

So with tu, Italian normally uses:

• non + infinitive

That is why you get Non lasciare.

Lasciare is the infinitive of the verb, meaning to let, to allow, or sometimes to leave depending on context.

In this sentence, because it is a negative tu command, the infinitive is used after non:

• Non lasciare... = Don’t let...

So even though lasciare is normally the dictionary form, here it is also the correct command form.

Here che means that and introduces a subordinate clause:

• Non lasciare che...
• Don’t let that... → more natural English: Don’t let ...

So:

• che il cane morda il cuscino = that the dog bite the cushion

Italian often uses lasciare che + clause, where English may simply say let + noun + verb.

Because after lasciare che, Italian normally uses the subjunctive.

So:

• morde = indicative, he/she/it bites
• morda = present subjunctive, he/she/it bite

In this structure:

• lasciare che + subjunctive

So:

• Non lasciare che il cane morda il cuscino.

This is one of the most important things to notice in the sentence.

It comes from the verb mordere = to bite.

Present subjunctive of mordere:

• che io morda
• che tu morda
• che lui/lei morda
• che noi mordiamo
• che voi mordiate
• che loro mordano

Since the subject is il cane = the dog (third person singular), the correct form is morda.

Because Italian usually needs an article in situations where English may or may not use one.

• il cane = the dog
• cane by itself is not normally used here

The sentence is talking about a particular dog, or at least a dog already understood in context, so il cane is natural.

Cuscino is a masculine singular noun, so it takes il:

• il cuscino = the cushion / the pillow

This is just normal noun + article agreement:

• masculine singular → il
• feminine singular → usually la

So the article is there because Italian generally uses articles with nouns more consistently than English.

Yes, that is closer to the Italian structure, but it is not natural English.

Italian structure:

• Non lasciare che il cane morda il cuscino

Literal idea:

• Don’t allow that the dog bite the cushion

Natural English:

• Don’t let the dog bite the cushion

So the Italian grammar uses che + subjunctive, while English prefers object + bare verb after let.

In standard Italian, yes, normally.

Examples:

• Lascio che lui parli. = I let him speak.
• Non lasciare che lei entri. = Don’t let her come in.
• Lasciavano che i bambini giocassero. = They let the children play.

So lasciare che is a very common trigger for the subjunctive.

Yes. A common alternative is with fare + infinitive:

• Non fare mordere il cuscino al cane.

This also means something like Don’t let the dog bite the cushion, though the structure feels different.

Still, non lasciare che il cane morda il cuscino is perfectly normal and very useful because it clearly shows:

• a negative tu command
• lasciare che
• the subjunctive

So it is an excellent sentence for learning.

Here it means to let / to allow.

Lasciare can mean different things depending on context:

• lasciare qualcosa sul tavolo = to leave something on the table
• lasciare qualcuno parlare = to let someone speak
• lasciare che qualcuno faccia qualcosa = to let someone do something

In this sentence, it clearly means to allow:

• Non lasciare che il cane morda il cuscino.
• Don’t let the dog bite the cushion.

Because Italian generally does not use the English-style pattern let + object + bare infinitive in the same way.

English:

• Don’t let the dog bite the cushion

Italian usually prefers:

• Non lasciare che il cane morda il cuscino
• or another structure such as Non far mordere il cuscino al cane

So if you translate word-for-word from English, you can easily produce something unnatural. The che + subjunctive pattern is the key structure to learn here.