Το πλήθος είναι έξω.

Because πλήθος is a neuter singular noun, so it takes the neuter singular definite article το.

A useful thing to notice is that Greek noun endings do not always match what an English speaker might expect. Even though πλήθος ends in -ος, it is neuter, not masculine.

So:

• το πλήθος = the crowd / the multitude

Grammatically, πλήθος is singular.

Even though it refers to many people as a group, Greek treats it as one collective unit, so the verb is singular too:

• Το πλήθος είναι... = The crowd is...

This is similar to English the crowd is in standard singular agreement.

Είναι is the present-tense form of είμαι meaning to be.

In this sentence, it means is.

A useful detail: είναι is used for both:

• he/she/it is
• they are

So you understand the number from the subject:

• Το πλήθος είναι έξω. = singular subject, so is
• Οι άνθρωποι είναι έξω. = plural subject, so are

Έξω is an adverb.

Here it means outside. Since it is an adverb, it does not change form for gender, number, or case.

So:

• έξω stays έξω no matter what the subject is

Examples:

• Είμαι έξω. = I am outside.
• Είναι έξω. = He/She/It is outside. / They are outside.
• Το πλήθος είναι έξω. = The crowd is outside.

Yes. Έξω can mean outside or out, depending on context.

For example:

• Είμαι έξω. = I am outside.
• Βγες έξω. = Go out.

In your sentence, the meaning is clearly outside.

This is the most neutral, standard Greek word order here:

• subject + verb + adverb

So:

• Το πλήθος = subject
• είναι = verb
• έξω = adverb

Greek word order is more flexible than English, so other orders are possible for emphasis, such as:

• Έξω είναι το πλήθος.
• Το πλήθος έξω είναι. (less neutral, more marked)

But Το πλήθος είναι έξω is the normal default sentence.

Usually, no, not in an ordinary sentence like this.

Greek uses the definite article very often, much more than English does. If you are talking about the crowd, you normally say:

• Το πλήθος

Without the article, Πλήθος είναι έξω sounds unnatural in most everyday contexts, unless it is some special style such as a headline, poetry, or very compressed speech.

Not exactly.

Πλήθος often means:

• crowd
• multitude
• large number

So it can be used both for a mass of people and more generally for a great number of things.

Examples:

• Το πλήθος είναι έξω. = The crowd is outside.
• Υπήρχε πλήθος κόσμου. = There was a crowd / a large number of people.
• Πλήθος προβλημάτων = a multitude of problems / many problems

So the exact English translation depends on context.

Because in Greek, grammatical gender and natural gender are not the same thing.

A noun can refer to people but still be grammatically neuter. Here, πλήθος is a collective noun, and its grammatical gender is neuter.

That means it takes:

• neuter article: το
• singular agreement: είναι

Even if the crowd is made up of men and women, the noun itself stays neuter.

A simple learner-friendly pronunciation guide is:

• Το ≈ toh
• πλήθος ≈ PLEE-thos
• είναι ≈ EE-ne
• έξω ≈ EK-so

A few pronunciation notes:

• ή in πλήθος is pronounced like ee
• θ sounds like the th in think
• ει in είναι is pronounced ee
• ξ sounds like ks
• The accent marks show which syllable is stressed:
  • πλήθος → stress on πλή-
  • είναι → stress on εί-
  • έξω → stress on έ-

It is neutral and standard. There is nothing especially formal or informal about it.

You could use it in normal speech, writing, or description.

Yes, depending on the situation.

Other common possibilities might be:

• ο κόσμος = the people / the crowd
• η ομάδα = the group
• ο όχλος = the mob (more negative)

For example:

• Ο κόσμος είναι έξω. = The people / the crowd are outside.
• Η ομάδα είναι έξω. = The group is outside.

But το πλήθος specifically gives the sense of a crowd or a large gathered mass.