Learn French: Elle est née en deux mille trois. - She was born in two thousand and three.

Because naître (to be born) uses être as its helping verb in the past tense. When a verb uses être, its past participle must agree with the subject. Since Elle is feminine, we add an e to make it née.

Why do we use en before the year?

In French, the preposition en is used to say 'in' for years, months, and seasons (except spring). So 'in 2003' is simply en deux mille trois.

Why doesn't mille have an s since there are two of them?

The French word mille (thousand) is invariable, meaning it never takes an s in the plural. Whether it is two thousand or ten thousand, it remains mille.

The English translation says 'two thousand and three'. Why is there no et in the French sentence?

In French numbers, you do not use et (and) to connect thousands or hundreds to the rest of the number. You only use et for tens ending in one, like vingt et un. So it is just deux mille trois.