Learn Spanish (Spain): El curso comienza el lunes.

In Spanish, you do not use a preposition to say 'on' a certain day of the week. Instead, you simply use the masculine singular article el before the day. If you wanted to say 'on Mondays' (every Monday), you would use los lunes.

What is the difference between comenzar and empezar?

Both verbs mean 'to begin' or 'to start' and they are generally interchangeable. Comenzar is often considered slightly more formal or elegant, but you will hear native speakers use both frequently in everyday conversation.

Is comenzar a regular verb?

It is a stem-changing verb. The e changes to ie in the present tense for most forms, which is why we see comienza here. However, remember that the nosotros and vosotros forms keep the regular stem: comenzamos and comenzáis.

Lunes ends in an 's'. Is it plural here?

No, it is singular here, which we know because it uses the singular article el. The Spanish words for Monday through Friday all end in 's' natively (lunes, martes, etc.). They stay exactly the same when plural, so you just change the article to los to make them plural.