Marie met en place le système.

Here met en place is the expression mettre en place, which commonly means to set up, to put in place, or to implement.

So Marie met en place le système means that Marie is establishing or installing the system, not just physically putting something somewhere.

Because the subject is Marie, which is third person singular: she.

The present tense of mettre goes like this:

• je mets
• tu mets
• il / elle / on met
• nous mettons
• vous mettez
• ils / elles mettent

So with Marie, you use elle met, therefore:

• Marie met

Yes. Mettre en place is a very common fixed expression in French.

It is often used with things like:

• mettre en place un système
• mettre en place une stratégie
• mettre en place des règles
• mettre en place une procédure

Even though the words literally suggest put in place, the full expression often has the broader meaning of set up or implement.

Because mettre en place and placer are not exactly the same.

• placer often means to place or to position
• mettre en place often means to establish, set up, or organize so it is ready to function

So Marie place le système would usually sound more physical or less idiomatic in many contexts. For systems, plans, procedures, and structures, mettre en place is often the natural choice.

Because mettre en place functions as a verbal expression, and the object often comes after it:

• mettre en place un système
• mettre en place un projet
• mettre en place des mesures

So in this sentence:

• Marie = subject
• met en place = verb expression
• le système = direct object

A useful pattern to remember is:

• Subject + mettre en place + object

Le système is the direct object of the verb expression.

It answers the question what is Marie setting up?

• Marie sets up the system
• the system = le système

The article le means the, and système is a masculine singular noun, so le système means the system.

Yes. Met is the present tense form of mettre.

So depending on context, the sentence could be translated as:

• Marie sets up the system
• Marie is setting up the system

French present tense often covers both the simple present and the present progressive idea that English sometimes separates.

Not exactly word for word.

Literally, en place can mean something like in place, but in mettre en place, the whole expression is idiomatic. You should learn it as one chunk:

• mettre en place = to set up / implement

So it is better not to translate each word separately every time.

Because Marie is already the subject.

In English, if you use a name, you do not also say she:

• Marie sets up the system.

French works the same way:

• Marie met en place le système.

You would only use elle if you replaced Marie:

• Elle met en place le système.

You would use the direct object pronoun le:

• Marie le met en place.

This is useful because it shows how object pronouns work in French: the pronoun goes before the conjugated verb.

So:

• Marie met en place le système.
• Marie le met en place.

Système is pronounced roughly like sees-TEM or sis-TEM, depending on how close you want to get in English.

The accent grave in è shows that the vowel is pronounced more like eh:

• système → the stressed-sounding vowel is like -tèm

Also note that y in French often sounds like an ee sound here.

So the word does not sound like English system.

No. Very often it is used for abstract things too, and that is one of its most common uses.

For example:

• mettre en place un plan = set up a plan
• mettre en place une politique = implement a policy
• mettre en place une méthode = put a method in place

So in Marie met en place le système, the sentence could refer to a literal system, a technical system, or an organizational system depending on context.