Learn Spanish (Latin America): Hay ochocientos estudiantes en el estadio. - There are eight hundred students in the stadium.

Ochocientos is made by simply combining ocho (eight) and cientos (hundreds) into a single word. Most hundreds in Spanish are formed this way, like doscientos (200) or trescientos (300).

Would ochocientos change if we were talking about an all-female group of students?

Yes! Unlike numbers 2 through 99, the hundreds from 200 to 999 change to match the gender of the noun they describe. If it were a group of only female students, it would be ochocientas estudiantes. We use the masculine ochocientos here for a mixed group or when the gender isn't specified.

Why does the sentence use hay for "there are" instead of a plural verb?

Hay is a special, unchangeable word that means both "there is" and "there are" to state that something exists. Even though there are 800 students, hay never takes a plural form.

Why does estadio start with an 'e' when the English word 'stadium' starts with an 's'?

In Spanish, words never start with an 's' followed immediately by another consonant (like st- or sp-). They always add an 'e' at the beginning. When you say it, make sure to pronounce the first syllable clearly: es-ta-dio.