Marie ne sait pas ; moi non plus.

French standard negation is usually built with two parts: ne ... pas.

So:

• Marie sait = Marie knows
• Marie ne sait pas = Marie does not know

In this sentence, ne comes before the verb and pas comes after it.

Sait is the third-person singular present form of the verb savoir, which means to know.

So:

• je sais = I know
• tu sais = you know
• il/elle sait = he/she knows

Because Marie is third-person singular, French uses sait.

French uses the stressed pronoun moi, not the subject pronoun je, in this kind of short reply.

So:

• je is used as the normal subject before a verb: je sais
• moi is used for emphasis, contrast, or short standalone expressions: moi aussi, moi non plus

That is why moi non plus is correct, while je non plus is not.

Non plus means neither or not ... either, depending on how you translate it in English.

It is used to agree with a negative statement.

Examples:

• Je ne comprends pas. — Moi non plus.
  = I do not understand. — Me neither / Neither do I.

• Marie ne sait pas ; moi non plus.
  = Marie does not know; neither do I.

Because the first statement is negative.

Use:

• aussi after an affirmative idea

  • Marie sait. Moi aussi.
    = Marie knows. I do too.

• non plus after a negative idea

  • Marie ne sait pas. Moi non plus.
    = Marie does not know. Neither do I.

So moi aussi would not fit this sentence.

Yes. Je ne sais pas non plus is grammatically correct and means I do not know either.

The difference is mostly style:

• Moi non plus is a short, natural reply
• Je ne sais pas non plus is fuller and more explicit

In conversation, moi non plus is very common when you are simply agreeing with someone else's negative statement.

Because French often leaves out repeated material when it is already clear from context.

In full, the idea is something like:

• Marie ne sait pas ; moi, je ne sais pas non plus.

But French commonly shortens this to:

• Marie ne sait pas ; moi non plus.

The omitted verb is understood from the first clause.

Not especially for grammar. It mainly separates two closely related parts.

You could also often see:

• Marie ne sait pas, moi non plus.
• Marie ne sait pas. Moi non plus.

The semicolon gives a slightly more formal or carefully written feel, but the meaning stays the same.

In standard written French, yes: Marie ne sait pas.

In informal spoken French, people often drop ne, so you may hear:

• Marie sait pas
• Moi non plus

This is very common in speech, but learners should still know that ne ... pas is the standard written form.

Here, plus is usually pronounced with the final s sound: pluss.

So moi non plus sounds roughly like:

• mwa non pluss

This is important because plus is not always pronounced the same way in French. In non plus, the final s is normally heard.

Yes. English has a few natural ways to translate it:

• Me neither
• Neither do I
• I do not either or I do not either are possible but less natural

So moi non plus is one French expression that can match several natural English responses.

Yes, a little. The sentence sets up a comparison between two people:

• Marie ne sait pas = Marie does not know
• moi non plus = I do not know either

Using moi highlights as for me, I do not either. It is a very natural way to contrast or align yourself with someone else.