Learn Spanish (Latin America): El estudiante escribe un mensaje en español.

Escribe comes from the regular -ir verb escribir (to write). Because the subject is El estudiante (a third person, like he or she), we drop the -ir ending and add -e. If you were saying 'I write', it would be escribo.

Estudiante ends in -e. How do we know if it is a masculine or feminine word?

Nouns ending in -e can be masculine or feminine. For people, the word estudiante stays exactly the same for both men and women. You can tell this specific student is male because it uses the masculine word for 'the', which is El. A female student would be La estudiante.

Is mensaje a masculine or feminine word?

It is masculine, which is why it uses the word un (a) instead of una. A helpful tip: most Spanish nouns that end in -aje are masculine.

Why is it just en español and not en el español?

When you say a message is written 'in Spanish', you just use the word en directly before the language. You do not need to add el (the) in this situation.