Learn Finnish: Harjoittelu alkaa maanantaina. - The internship begins on Monday.

Harjoittelu is formed from the verb harjoitella (to practice) using the -elu suffix, which describes an ongoing process or period of time, making it the standard word for an internship. Harjoitus usually means a single exercise, drill, or a specific practice session.

Why do we use alkaa here instead of aloittaa?

Alkaa is intransitive, meaning the subject itself begins or starts, like the internship in this sentence. Aloittaa is transitive and requires an active subject doing the starting. For example, you would use it if you said Minä aloitan harjoittelun (I start the internship).

Why does maanantai have the -na ending here?

The -na ending is the essive case. While the essive case is often used to mean 'as a role or state', it is also the standard grammatical rule in Finnish to express 'on a certain day of the week'. Therefore, maanantaina simply means 'on Monday'.