Learn Spanish (Latin America): Él trabaja constantemente en el proyecto.

The adjective here is constante (constant). Adjectives that end in -e don't have separate masculine and feminine forms — they stay exactly the same for both genders. Since there is no special feminine form ending in 'a', you just take the standard word constante and add -mente to make constantemente.

Why does Él have an accent mark here?

The accent mark is there to tell the difference between él (he) and el (the). If you forgot the accent, it would look like you were saying 'The works constantly', which wouldn't make sense!

In English we say we work 'on' a project. Why does Spanish use en instead of a word for 'on' like sobre?

While sobre often means 'on' when talking about physical location (like a book on a table), Spanish uses en (in/on/at) for being involved in an activity or task. So, to work 'on' a project is always trabajar en un proyecto.

Could I put constantemente at the very end of the sentence instead?

Yes! Spanish word order is quite flexible. You can absolutely say Él trabaja en el proyecto constantemente. However, putting the adverb right after the verb it describes, like trabaja constantemente, is the most common and natural way to say it.