Non accusare Marco senza motivo.

Because this is a negative command addressed to tu.

In Italian, the negative imperative for tu is formed with:

non + infinitive

So:

• Accusa Marco! = Accuse Marco!
• Non accusare Marco! = Don’t accuse Marco!

This is one of the first command patterns learners notice because it is different from the positive command.

Formally, accusare is the infinitive form of the verb, but in this sentence it is being used as a command because of the negative imperative pattern:

non + infinitive

So the sentence means Don’t accuse Marco without reason, not simply to not accuse Marco.

Italian usually omits subject pronouns when they are not needed.

So instead of saying:

Tu non accusare Marco senza motivo

Italian normally just says:

Non accusare Marco senza motivo

The verb form and the context make it clear that the speaker is talking to you.

Because Marco is the direct object of accusare.

In Italian, when a verb takes a direct object, you normally do not add a preposition just because the object is a person.

So:

• accusare Marco = to accuse Marco
• not accusare a Marco

This is different from some other languages where a preposition or personal marker may appear before people.

Literally, senza motivo means without reason.

In natural English, it often corresponds to:

• without reason
• for no reason
• without any good reason

It is a very normal Italian expression.

Both are possible, but senza motivo is a very common, natural expression.

Compare:

• senza motivo = without reason / for no reason
• senza un motivo = without a reason

The version without the article can sound a bit more fixed and idiomatic. In many common expressions after senza, Italian often leaves the noun bare.

Yes. Senza una ragione is grammatically correct and understandable.

However, senza motivo is especially common and idiomatic in this context. It is a compact, natural way to say without reason or for no reason.

Non normally goes before the verb or the verbal form it negates.

Here it comes before accusare:

Non accusare Marco senza motivo.

That is the standard position for negation in Italian.

The negative command changes depending on who you are talking to.

• tu: Non accusare Marco senza motivo.
• Lei (formal singular): Non accusi Marco senza motivo.
• voi (plural): Non accusate Marco senza motivo.

So the special non + infinitive pattern is specifically for the informal tu form.

It is pronounced roughly ak-koo-ZA-re.

A few useful details:

• the cc in accusare sounds like a strong k sound: akk-
• the stress falls on -sa-: accuSAre

So the whole sentence sounds roughly like:

non ak-koo-ZA-re MAR-ko SEN-tsa mo-TEE-vo

Mostly yes. It is a close equivalent of English to accuse.

Common pattern:

• accusare qualcuno di qualcosa = to accuse someone of something

For example:

• Ha accusato Marco di mentire. = He accused Marco of lying.

In your sentence, only the person being accused appears:

• Non accusare Marco... = Don’t accuse Marco...

Yes.

By itself, Non accusare Marco senza motivo is a straightforward warning or instruction. Its tone depends on context, voice, and punctuation.

For example:

• with a calm tone, it can sound like advice
• with a sharp tone, it can sound like a reprimand

If you wanted to soften it, Italian might add words like:

• per favore = please
• non accusare Marco senza motivo, per favore

So the grammar gives the command, but the context gives the tone.