La soucoupe est pleine.

Because soucoupe is a feminine noun in French. Feminine singular nouns normally take la.

So:

• la soucoupe = the saucer

If the noun were masculine, you would usually use le instead.

French noun gender is grammatical, so it is something you usually have to learn with the noun.

Because pleine is an adjective agreeing with the feminine noun soucoupe.

Since soucoupe is feminine singular, the adjective must also be feminine singular:

• masculine: plein
• feminine: pleine

Compare:

• Le bol est plein.
• La soucoupe est pleine.

This is a very common pattern in French: adjectives often change form to match the gender and number of the noun they describe.

Est is the third-person singular form of the verb être (to be).

The subject is la soucoupe = the saucer, which is a third-person singular subject, so French uses est:

• je suis = I am
• tu es = you are
• il/elle est = he/she is

So:

• La soucoupe est pleine = The saucer is full.

It is an adjective.

In this sentence, pleine describes the state of the saucer. It comes after the verb être, so it functions like full in English:

• The saucer is full.

This is called a predicate adjective: it comes after to be and describes the subject.

A simple pronunciation guide is:

la soo-koop eh plen

A bit more accurately:

• La → la
• soucoupe → soo-koop
• est → eh
• pleine → roughly plen

The final -e in pleine is not pronounced as a separate sound.

Usually, une soucoupe means a saucer, the small dish under a cup.

But in other contexts, it can also mean a flying saucer:

• une soucoupe volante = a flying saucer

So the word can have more than one meaning, just like many English words. Context tells you which meaning is intended.

Because pleine is not a noun here; it is an adjective.

In La soucoupe est pleine:

• la soucoupe = the subject
• est = the verb
• pleine = the adjective describing the subject

So French works like English here:

• The saucer is full not
• The saucer is the full

Yes, you can, but it is not exactly the same in tone.

• pleine = full
• remplie = filled / full

La soucoupe est pleine is the most natural simple way to say the saucer is full.

La soucoupe est remplie can sound a little more like it has been filled, or that you are focusing on the result of filling it.

Also, if you want to say what it is full of, French often uses:

• La soucoupe est pleine d’eau. = The saucer is full of water.

Yes. This is a very standard sentence pattern in French:

subject + être + adjective

So:

• La soucoupe = subject
• est = is
• pleine = full

This matches English quite closely:

• The saucer is full.

Yes. Both the article and the adjective would change.

Singular:

• La soucoupe est pleine. = The saucer is full.

Plural:

• Les soucoupes sont pleines. = The saucers are full.

Notice the changes:

• la → les
• est → sont
• pleine → pleines

French often shows number and gender agreement in several parts of the sentence.