Un milion este mult.

It can be understood as both.

In Romanian, un is:

• the masculine/neuter singular indefinite article: a / an
• the numeral one

So un milion can mean a million or one million. In this sentence, English often uses A million is a lot, but One million is a lot also matches the Romanian.

Because milion is a neuter noun in Romanian.

Neuter nouns in Romanian behave:

• like masculine in the singular
• like feminine in the plural

So you get:

• un milion = one/a million
• două milioane = two million

That is why o milion is incorrect.

Yes. Milion is a noun, not just a number word in some special separate category.

It behaves like other nouns:

• singular: milion
• plural: milioane
• definite form: milionul = the million

Examples:

• un milion
• două milioane
• milionul acesta

Because de is used when milion is followed by another noun.

For example:

• un milion de euro
• un milion de oameni

But in Un milion este mult, the word milion stands on its own, so no de is needed.

Este means is.

It is the:

• 3rd person singular
• present tense
• of the verb a fi = to be

So:

• un milion = a million
• este = is
• mult = a lot / much

Yes. That is very common.

E is the short form of este in everyday Romanian:

• Un milion este mult. = a bit more full or careful
• Un milion e mult. = very natural in speech

Both are correct.

Because the subject is singular: un milion.

Just like in English:

• A million is a lot

Romanian also uses a singular verb here because milion is one grammatical unit in the singular.

Here mult means a lot, much, or a large amount.

It does not mean big in a physical sense.
So Romanian uses mult, not mare, to talk about quantity here.

Compare:

• mult = much / a lot
• mare = big / large

So Un milion este mult means A million is a lot, not A million is big.

Because mult agrees with milion, and milion is singular neuter.

As a neuter noun in the singular, milion takes the same form as a masculine singular noun. So the correct agreeing form is:

• mult

Very roughly:

• masculine singular: mult
• feminine singular: multă
• masculine plural: mulți
• feminine/neuter plural: multe

Since un milion is singular, mult is the right form.

That is exactly how Romanian neuter works.

Romanian neuter nouns:

• use masculine forms in the singular
• use feminine forms in the plural

So:

• singular: un milion, mult
• plural: două milioane, multe

This masculine-in-the-singular / feminine-in-the-plural pattern is one of the main things to remember about Romanian neuter nouns.

Yes. It is the neutral, standard order:

• Un milion = subject
• este = verb
• mult = predicate/complement

Romanian word order is somewhat flexible, but Un milion este mult is the most straightforward and natural way to say it.

Yes.

Romanian, like English, can use un milion by itself when the exact thing is not important or is already understood.

So this sentence can mean something general like:

• A million is a lot
• One million is a lot

If you want to be specific, you add de + noun:

• Un milion de lei este mult.
• Un milion de oameni este mult.

A rough English-style guide is:

oon mee-lee-ON YES-teh mult

A few notes:

• milion has the stress on the last syllable: mee-lee-ON
• este is often pronounced like YES-te
• mult is short and compact, close to moolt, but with a clearer u

So the full sentence sounds roughly like:

oon mee-lee-ON YES-teh moolt